Images affect your SEO in two distinct ways. They directly impact your page speed — which is a confirmed ranking signal — and they appear as a separate search surface in Google Images, which can drive significant additional traffic if your images are properly optimised. Most websites handle neither dimension well.

Quick Answer: To optimise images for SEO: use descriptive, keyword-relevant alt text on every meaningful image; convert images to WebP or AVIF format to reduce file size without quality loss; specify width and height attributes to prevent layout shift; use lazy loading for images below the fold; and name your image files descriptively before uploading. Together these improve both page speed rankings and image search visibility.

What is alt text and how do you write it well for SEO?

Alt text (alternative text) is a written description of an image that serves three purposes: it helps visually impaired users using screen readers understand what an image shows, it appears in place of a broken image, and it tells Google what the image depicts so it can appear in relevant image searches.

How to write effective alt text:

  • Be specific and descriptive — "Website performance dashboard showing 2.1 second load time for London e-commerce store" is good alt text. "Image" or "photo" is not.
  • Include relevant keywords naturally — if you have a broken link checker tool, the alt text "free broken link checker scanning a website for 404 errors" naturally includes relevant terms without stuffing
  • Keep it under 125 characters — screen readers typically truncate after this length
  • Do not start with "image of" or "picture of" — Google and screen readers already know it is an image. Describe what it shows, not what it is.
  • Leave alt text empty (alt="") for purely decorative images — icons, decorative borders, and background patterns do not need alt text and adding it creates noise for screen readers

Does Google read alt text as a ranking signal?

Yes — Google uses alt text to understand image content for Google Images rankings, and it also contributes to the page's overall keyword relevance signals. A page about broken links that contains an image with alt text describing a broken link checker tool has an additional relevance signal reinforcing the page's topic. Alt text is also used by Google's AI systems to understand the relationship between images and surrounding content.

What is the best image format for SEO in 2026?

WebP and AVIF are the recommended formats for web images in 2026. Both provide significantly smaller file sizes than JPEG and PNG while maintaining equivalent or better visual quality.

  • WebP — typically 25–35% smaller than equivalent JPEG. Supported by all modern browsers. The current safe choice for broad compatibility.
  • AVIF — typically 40–50% smaller than JPEG. Superior compression but slightly slower encoding. Excellent browser support in 2026 and the best choice for performance-critical pages.
  • JPEG — still acceptable for photographs where WebP/AVIF are not available, but larger file sizes harm page speed
  • PNG — use only for images requiring transparency or lossless quality (logos, screenshots, icons). Avoid for photographs.
  • SVG — ideal for icons, logos, and simple graphics. Resolution-independent and typically very small file sizes.

Converting your images to WebP or AVIF is the single fastest Core Web Vitals improvement for most image-heavy pages. Test your current scores with our page speed tool before and after conversion to measure the impact.

How do image dimensions affect SEO and Core Web Vitals?

Always specify explicit width and height attributes on your <img> tags:

<img src="image.webp" alt="Description" width="800" height="600">

Without these attributes, the browser does not know how much space to reserve for the image before it loads. This causes Cumulative Layout Shift (CLS) — content visibly jumping as images load in. CLS is one of Google's three Core Web Vitals ranking signals. Specifying dimensions is a simple, zero-cost CLS fix that most sites have not implemented.

What is lazy loading and should you use it for SEO?

Lazy loading is a technique where images below the visible screen area (below the fold) are only loaded when the user scrolls down toward them, rather than all loading immediately when the page first opens. This improves initial page load time because the browser only downloads what the user can currently see.

The correct implementation in 2026:

<!-- Images above the fold (hero image) - load immediately -->
<img src="hero.webp" alt="Description" fetchpriority="high">

<!-- Images below the fold - lazy load -->
<img src="content-image.webp" alt="Description" loading="lazy">

Important: do not lazy load your hero image or any image visible when the page first loads. Lazy loading your LCP element — the largest visible image — directly worsens your LCP score.

How do you name image files for SEO?

File names are a secondary but real signal for image search rankings. Google reads file names to understand image content before processing the image itself.

  • Good: broken-link-checker-tool.webp, london-plumber-emergency-service.jpg
  • Bad: IMG_4829.jpg, photo.png, image1.webp, DSC00234.jpg

Use hyphens to separate words (not underscores — Google treats underscores as word joiners, not separators). Include your primary keyword or a clear description of what the image shows. Keep file names under 5–6 words to stay descriptive without keyword stuffing.

Should you add structured data to images?

Yes — for specific content types, ImageObject schema enhances how your images appear in search results. Product images benefit from Product schema that includes the image. Recipe images benefit from Recipe schema. Article images benefit from Article schema with a named image property.

For photography or visual portfolio sites, ImageObject schema with explicit creator, dateCreated, copyrightYear, and contentUrl properties helps Google attribute and display your work accurately in image search results.

Summary

  • Write specific, descriptive alt text under 125 characters with relevant keywords naturally included
  • Convert images to WebP or AVIF format — typically 25–50% smaller than JPEG with equivalent quality
  • Always specify width and height attributes to prevent Cumulative Layout Shift (a Core Web Vitals signal)
  • Use loading="lazy" for below-the-fold images; use fetchpriority="high" for your hero/LCP image
  • Name image files descriptively using hyphens before uploading
  • Test page performance improvements after image optimisation with our speed tool

Continue reading: What is schema markup and how does it help your SEO rankings?