When talking about technical SEO, one immediately thinks of meta tags, redirects, and schema markup. Images come last — if they come at all. This is a strategic mistake. Images are the primary driver of page weight, the primary cause of Core Web Vitals degradation, and a channel for massive traffic through Google Images (which accounts for 20-25% of total search traffic).
In 2026, an additional dimension is added: multimodal LLMs (GPT-4o, Gemini, Claude) "see" images and read alt texts. Your visual content is no longer invisible to AI.
The real impact of images on SEO
The data from HTTP Archive (January 2026) is unambiguous:
| Metric | Median value | SEO impact |
|---|---|---|
| Total image weight per page | 1.2 MB | Directly impacts LCP |
| Number of images per page | 26 | Additional HTTP requests |
| Images without alt text | 33% | Loss of accessibility and SEO context |
| Images in legacy formats (JPEG/PNG) | 58% | 30-50% heavier than WebP/AVIF |
| Images without lazy loading | 41% | Unnecessarily long initial load |
These figures show both the scale of the problem and the scale of the opportunity. Optimising your images is often the fastest and most impactful way to improve your Core Web Vitals.
Alt text: the definitive guide
The alt attribute (alternative text) has three functions: accessibility (screen readers for visually impaired users), SEO (giving Google context about the content of the image), and in 2026, AI readability (multimodal LLMs use the alt text as a descriptive caption).
The rules for a good alt text
- Descriptive: describe what the image shows, not what it is. "Graph showing the evolution of organic traffic from 2024 to 2026 with a 45% increase" > "graph"
- Concise: maximum 125 characters. Beyond that, screen readers cut off.
- Contextual: integrate the context of the page. If the article is about technical SEO, mention it in the alt text.
- No keyword stuffing: "alt text SEO images optimisation SEO alt text" is penalised. Integrate the keyword naturally.
- No "image of": screen readers already announce "image". Writing "image of..." is redundant.
Concrete examples
Bad: alt="" (empty) or alt="image" or alt="photo1.jpg"
Mediocre: alt="comparison table"
Good: alt="Comparison table of WebP, AVIF and PNG image formats with compression gain for each format"
According to Leonie Watson, web accessibility expert based in London: "Alt text is a bridge between the visual and the non-visual. In 2026, that bridge also serves AI systems that 'read' your pages. A precise alt text benefits both humans and machines."
Image formats in 2026: WebP, AVIF, SVG
WebP
The reference format in 2026. Supported by all modern browsers (99.5% coverage according to Can I Use). Offers 25-35% better compression than JPEG with equivalent visual quality. Supports transparency (replacing PNG) and animation (replacing GIF).
AVIF
The next-generation format. 20% better compression than WebP, but encodes more slowly and is supported by 93% of browsers in 2026. Ideal for sites with high-quality images (portfolio, premium e-commerce). Use WebP as a fallback.
SVG
For logos, icons, and vector illustrations. Minimal size, infinite resolution, and modifiable with CSS. Only use SVG for vector elements — never for photos.
Compression and performance
The winning trio: format + dimensions + compression
Format: WebP or AVIF. Ban PNG for photos (10x heavier than WebP at equivalent quality).
Dimensions: never serve an image larger than its display size. An image of 3000px displayed at 600px wastes 80% of bandwidth. Use the srcset attribute to serve images adapted to each screen size.
Compression: WebP at quality 80 offers an excellent quality/weight compromise. Below 70, artefacts become visible. Above 90, the quality gain is imperceptible but the weight increases significantly.
Lazy loading
The loading="lazy" attribute delays the loading of images not visible on screen. In 2026, this is a native HTML standard — no JavaScript needed. Exception: images above the fold (hero, logo) must load immediately — add loading="eager" and fetchpriority="high" for the LCP element.
Images and AI visibility: the new challenge
In 2026, multimodal LLMs no longer just read text. GPT-4o, Gemini 2.0, and Claude 3.5 analyse images. When a RAG system crawls your page, it "sees" your visuals and reads your alt texts to understand the full context.
The implications are concrete: a page with well-described infographics (detailed alt text, visible caption, ImageObject schema markup) provides more actionable information to LLMs than a page with decorative images without descriptions. Your visuals become a competitive advantage for AI visibility.
The recommendation: for each informative image (graph, infographic, screenshot, diagram), write an alt text that describes the key data, not just the type of visual. "Graph showing that 68% of European sites have at least 5 technical errors (source Ryte 2025)" is infinitely more useful for an LLM than "statistics graph".
Complete optimisation checklist
Alt text and SEO (8 points)
- All informative images have an alt text
- Decorative images have
alt=""(empty, not absent) - Alt texts are descriptive and contextual (no keyword stuffing)
- File names are descriptive (not "IMG_4532.jpg")
- Images are in a relevant context (paragraph, section)
- Infographics have an accessible text version
- Product images have an ImageObject schema
- Captions (
<figcaption>) are used when relevant
Performance (7 points)
- WebP or AVIF format for all photos
- Dimensions adapted to display (no oversizing)
srcsetattribute for responsive images- Compression at 80% for WebP
- Lazy loading on images below the fold
- Hero image with
fetchpriority="high" - Explicit
widthandheightattributes (avoids CLS)
For a global view of performance, see our guide on Core Web Vitals, and for complete page optimisation, our guide on meta tags.
FAQ
Do AI-generated images have a negative impact on SEO?
Google has stated that AI-generated images are treated like any other image. What matters is the relevance and added value for the user. However, generic stock images (AI or not) provide no SEO value. Prioritise informative visuals (infographics, diagrams, screenshots) that genuinely enrich the content.
How many images per blog article are recommended?
There is no absolute rule. The right answer: as many informative images as needed, zero unnecessary decorative images. For a 1,500-2,000 word article, 2 to 4 informative images (infographics, comparisons, screenshots) is a good balance. Each image should add value that text alone cannot convey.
Does the image file name have an impact on SEO?
Yes, it is a minor but real signal. Google uses the file name as an indicator of the image content. "image-format-comparison-webp-avif.webp" is more informative than "image-1.webp". Use hyphens, not underscores, and avoid special characters and accented letters.
Is a dedicated image sitemap necessary?
Google recommends including images in your standard XML sitemap with the <image:image> tag. A dedicated image sitemap is not required but can be useful for sites with many images (e-commerce, portfolio). It helps Google discover images that are not directly linked in the HTML.
Does a CDN (Content Delivery Network) impact image SEO?
A CDN improves image loading speed (and therefore Core Web Vitals), which has a positive indirect impact on SEO. However, make sure the images served from the CDN are crawlable (not blocked by robots.txt) and that cache headers are correctly configured. Modern CDNs like Cloudflare Images or imgix also offer automatic WebP/AVIF conversion.
How do I optimise images for Google Images?
Google Images accounts for 20-25% of search traffic. To appear there: use descriptive alt texts, meaningful file names, contextual captions, and ImageObject schema markup. Large images (minimum 1200px wide) have a greater chance of appearing in image results. And above all, the image must be on a page with relevant, well-ranked content.
Are your images dragging down your SEO?
Our experts audit and optimise your images to boost your performance and AI visibility.
Optimise my images
