How long should alt text be for SEO?

Alt text should ideally be between 5 and 15 words (roughly 80-125 characters). It should be descriptive, include your target keyword naturally, and accurately describe the image content. Avoid keyword stuffing — write for humans first and search engines second. Screen readers typically read the entire alt attribute, so keeping it concise ensures a good user experience for visually impaired visitors as well.

Does image file name really affect SEO rankings?

Yes, image file names are a confirmed ranking signal for Google Image Search. A descriptive, keyword-rich file name like 'handmade-leather-wallet-brown.jpg' tells search engines what the image depicts before they even analyze the visual content. Avoid generic names like 'IMG_2048.jpg' or 'screenshot-1.png'. Use hyphens to separate words, keep names lowercase, and make them human-readable. This small step can meaningfully improve your visibility in image search results.

What is the best image format for SEO in 2025?

WebP is currently the best all-around image format for SEO in 2025. It offers 25-35% smaller file sizes than JPEG at comparable quality and is supported by all major browsers. AVIF is even more efficient (up to 50% smaller) but has slightly less universal support. The recommended approach is to serve AVIF with a WebP fallback using the HTML element, and keep a JPEG or PNG as a final fallback for maximum compatibility.

SEO Images: Alt Text, File Naming & Full Visual Optimization Guide

Why Image SEO Matters More Than You Think

Images are not decoration. They are content — and search engines treat them that way.

Google Image Search accounts for over 22% of all web searches. Visual results now appear in standard SERPs through rich snippets, featured snippets, and the image pack. If your images aren’t optimized, you’re leaving a massive chunk of organic traffic on the table.

But image SEO isn’t just about rankings. It directly impacts:

Page load speed — unoptimized images are the #1 cause of slow websites
Core Web Vitals — LCP (Largest Contentful Paint) is often an image
Accessibility — screen readers depend on alt text to describe visuals
User experience — faster, clearer images reduce bounce rates

At Lueur Externe, a web agency based in the French Riviera with over 20 years of SEO and web development experience, we consistently see image optimization deliver some of the quickest performance and ranking wins for our clients. It’s often the lowest-hanging fruit on an SEO audit.

Let’s break down every aspect of image SEO — from file naming to structured data — so you can implement a complete visual optimization strategy.

File Naming: Your First SEO Signal

Why File Names Matter

Before an image is even uploaded to your site, its file name sends a signal to search engines. Google’s own documentation confirms that file names are used to understand image content.

Yet most websites are full of images named DSC_0042.jpg, image1.png, or Capture d'écran 2025-01-15.png. These tell search engines absolutely nothing.

Best Practices for Image File Names

Be descriptive: The file name should clearly describe the image content
Use hyphens (-) to separate words, not underscores or spaces
Keep it lowercase: Avoids potential server issues with case sensitivity
Include your target keyword naturally — don’t force it
Keep it concise: 3-7 words is the sweet spot
Use English or your target language consistently

File Naming Examples

❌ Bad File Name	✅ Good File Name
`IMG_3847.jpg`	`blue-running-shoes-nike-pegasus.jpg`
`photo1.png`	`organic-coffee-beans-close-up.png`
`screenshot 2025-03-10.png`	`prestashop-dashboard-order-management.png`
`header_bg_FINAL_v2.jpg`	`mountain-landscape-alps-sunset.jpg`
`product.jpeg`	`handmade-ceramic-mug-white-glaze.jpeg`

A simple rename before upload takes 10 seconds. Over hundreds of images, the cumulative SEO impact is significant.

Alt Text: The Cornerstone of Image SEO

What Is Alt Text?

The alt attribute (commonly called “alt text” or “alt tag”) is an HTML attribute applied to <img> elements. It serves two primary purposes:

Accessibility: Screen readers read alt text aloud to visually impaired users
SEO: Search engines use alt text as a primary signal to understand image content

When an image fails to load, the alt text is also displayed in its place — making it a usability feature as well.

How to Write Effective Alt Text

Here’s the formula that works:

Describe the image accurately + include the keyword naturally + keep it under 125 characters.

Let’s look at a concrete example. Imagine a product photo of a red leather handbag on a white background.

<!-- ❌ Bad: Empty alt text -->
<img src="product-12.jpg" alt="">

<!-- ❌ Bad: Keyword stuffing -->
<img src="product-12.jpg" alt="red handbag leather handbag buy red leather handbag cheap handbag">

<!-- ❌ Bad: Too generic -->
<img src="product-12.jpg" alt="handbag">

<!-- ✅ Good: Descriptive and natural -->
<img src="red-leather-crossbody-handbag.jpg" alt="Red Italian leather crossbody handbag with gold clasp on white background">

Alt Text Rules by Image Type

Not every image needs the same treatment:

Product images: Be very specific. Include color, material, brand, and product type.
Blog illustrations: Describe what’s happening in the image. Include the topic keyword.
Infographics: Summarize the key data point. Consider a longer alt or use longdesc.
Decorative images: Use an empty alt (alt="") so screen readers skip them. Think background textures, dividers, or purely aesthetic elements.
Logos: Use the company name, e.g., alt="Lueur Externe logo".
Charts and graphs: Describe the trend or conclusion, not just “chart”.

Common Alt Text Mistakes to Avoid

Starting with “Image of…” or “Photo of…” — screen readers already announce it’s an image
Leaving alt text blank on meaningful images
Using the same alt text for multiple different images
Writing alt text that doesn’t match the actual image content
Exceeding 125 characters regularly (some screen readers truncate)

Image Compression and Format Selection

The Speed-Quality Tradeoff

Every kilobyte matters. According to HTTP Archive data, images make up approximately 42% of an average web page’s total weight. A single unoptimized hero image can weigh 2-5 MB — enough to add seconds to your load time on mobile networks.

Google has been clear: page speed is a ranking factor. And since images are the heaviest assets on most pages, compression is non-negotiable.

Choosing the Right Format

Format	Best For	Transparency	Animation	Avg. Compression
JPEG	Photographs, complex images	❌	❌	Good
PNG	Graphics, logos, screenshots	✅	❌	Moderate
WebP	All-purpose (photos + graphics)	✅	✅	Excellent (25-35% smaller than JPEG)
AVIF	All-purpose, next-gen	✅	✅	Outstanding (up to 50% smaller than JPEG)
SVG	Icons, logos, simple illustrations	✅	✅	N/A (vector)
GIF	Simple animations	✅	✅	Poor

Serving Modern Formats with Fallbacks

The <picture> element lets you serve the most efficient format supported by each browser:

<picture>
  <source srcset="hero-banner.avif" type="image/avif">
  <source srcset="hero-banner.webp" type="image/webp">
  <img src="hero-banner.jpg" alt="Alpine landscape with snow-capped peaks at golden hour" 
       width="1200" height="630" loading="lazy">
</picture>

This approach ensures every visitor gets the smallest possible file while maintaining full compatibility.

Recommended Compression Tools

Squoosh (squoosh.app) — free, browser-based, excellent for manual optimization
ShortPixel — WordPress plugin with automatic compression on upload
TinyPNG/TinyJPG — simple API-based compression
ImageOptim (Mac) — desktop app for batch processing
Imagify — works with WordPress and offers WebP conversion

For PrestaShop stores — an area where Lueur Externe holds certified expertise — we typically implement server-side image optimization pipelines that automatically compress and convert product images on upload, ensuring consistent quality across catalogs with thousands of SKUs.

Image Dimensions and Responsive Delivery

Always Specify Width and Height

One of the most overlooked image optimizations is declaring width and height attributes directly in your HTML:

<img src="team-photo.webp" alt="Lueur Externe team at the Nice office" 
     width="800" height="533" loading="lazy">

Why does this matter? Without explicit dimensions, the browser doesn’t know how much space to reserve for the image. As it loads, the layout shifts — this is called Cumulative Layout Shift (CLS), one of Google’s three Core Web Vitals. High CLS directly hurts your rankings.

Responsive Images with srcset

Serving a 2400px-wide image to a mobile phone is wasteful. The srcset attribute lets browsers pick the right size:

<img srcset="product-400w.webp 400w,
            product-800w.webp 800w,
            product-1200w.webp 1200w"
     sizes="(max-width: 600px) 400px,
            (max-width: 1024px) 800px,
            1200px"
     src="product-1200w.webp"
     alt="Handcrafted ceramic vase with blue geometric pattern"
     width="1200" height="900"
     loading="lazy">

This can reduce image data transferred by 60-80% on mobile devices, dramatically improving load times and user experience.

Lazy Loading: Load What’s Visible

Lazy loading defers the loading of off-screen images until the user scrolls near them. It’s now natively supported in all major browsers with a single attribute:

<img src="photo.webp" alt="Description" loading="lazy">

Key rules for lazy loading:

Never lazy-load above-the-fold images (your hero image, logo, etc.) — these should load immediately
Do lazy-load everything below the fold: blog images, product grids, gallery items
Set loading="eager" explicitly on your LCP image to prioritize it
Consider using fetchpriority="high" on your most important image

<!-- Hero image: load immediately with high priority -->
<img src="hero.webp" alt="Main banner description" 
     loading="eager" fetchpriority="high" width="1440" height="600">

<!-- Below-fold image: lazy load -->
<img src="section-photo.webp" alt="Section description" 
     loading="lazy" width="800" height="450">

Structured Data for Images

Structured data helps search engines understand and feature your images in rich results. The most impactful schemas for images include:

Product Images

{
  "@context": "https://schema.org",
  "@type": "Product",
  "name": "Handmade Leather Wallet",
  "image": [
    "https://example.com/photos/wallet-front.webp",
    "https://example.com/photos/wallet-open.webp",
    "https://example.com/photos/wallet-back.webp"
  ],
  "description": "Hand-stitched brown leather bifold wallet with RFID protection"
}

Article Images

For blog posts, ensure your Article schema includes an image property. Google recommends providing images that are at least 1200px wide for inclusion in the Discover feed and rich results.

Image Sitemaps

Don’t forget to include images in your XML sitemap. This is especially critical for JavaScript-rendered sites where Google might miss images during crawling:

<url>
  <loc>https://example.com/products/leather-wallet</loc>
  <image:image>
    <image:loc>https://example.com/photos/wallet-front.webp</image:loc>
    <image:title>Handmade brown leather wallet front view</image:title>
  </image:image>
</url>

Most CMS platforms (WordPress, PrestaShop) can generate image sitemaps automatically with the right plugin or module configuration.

Content Surrounding the Image

Google doesn’t look at images in isolation. The text surrounding an image heavily influences how it’s understood and ranked.

Best practices:

Place images near the relevant text that describes the same topic
Use captions when appropriate — studies show captions are read 300% more than body text
Use descriptive headings above image sections
Ensure the page title and H1 align with the image topic
Internal link from image-heavy pages to related content

This contextual relevance is what separates images that rank from images that are invisible to search.

Image SEO Checklist

Here’s a quick-reference checklist you can use for every image on your site:

Measuring Image SEO Performance

You can’t improve what you don’t measure. Here are the tools to track your image optimization progress:

Google Search Console → Performance → Search type: Image — see impressions, clicks, and CTR for your images in Google Image Search
PageSpeed Insights — flags unoptimized images, missing dimensions, and LCP issues
Lighthouse — provides specific recommendations for image compression and format upgrades
Chrome DevTools → Network tab — inspect actual image sizes being transferred
Screaming Frog — audit missing alt text, oversized images, and broken image URLs at scale

A quick win: filter Google Search Console for image search queries where you rank positions 5-20. These are images that are almost visible — a few optimizations could push them onto the first row of results.

Conclusion: Images Are an SEO Powerhouse When Done Right

Image SEO is not a single task — it’s a system. From the moment you name a file to the structured data that wraps it on your page, every step contributes to how search engines discover, understand, and rank your visual content.

The good news? Most of your competitors are still uploading IMG_4092.jpg with empty alt text and no compression. That means proper image optimization is still a genuine competitive advantage.

To summarize the highest-impact actions:

Name files descriptively before uploading
Write meaningful alt text for every non-decorative image
Compress aggressively and use WebP/AVIF
Set dimensions to prevent layout shift
Implement lazy loading intelligently
Add structured data and image sitemaps

If you’d like expert guidance on implementing a complete image SEO strategy — or a full technical SEO audit for your WordPress or PrestaShop site — the team at Lueur Externe has been helping businesses across France and internationally optimize their web presence since 2003. As certified PrestaShop experts and AWS Solutions Architects, we bring both the SEO expertise and the technical infrastructure knowledge to ensure your site performs at its absolute best.

Ready to make your images — and your entire site — work harder for your business? Get in touch with Lueur Externe today for a personalized SEO consultation.