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Image SEO: findable images that do not wreck your page speed
Image SEO has two jobs: making images discoverable, and stopping them from slowing the page. Discoverability comes from descriptive filenames, real alt text and captions. Speed comes from modern formats, correct dimensions per viewport, and not lazy-loading the one image above the fold.
Alt text: write it for someone who cannot see the image
Alt text exists for screen reader users first. That is not a compromise with SEO — it produces exactly the text a search engine wants, because both are asking the same question: what is in this image and why is it here?
| Case | What to write |
|---|---|
| Informative image | What it shows, in a normal sentence fragment: "EICR certificate booking form on a mobile phone" |
| Image that is a link | Where the link goes, not what the picture is |
| Decorative image | alt="" — an empty attribute, so screen readers skip it. Never omit the attribute entirely |
| Image containing text | The text itself, verbatim |
| Product photo | Product name plus the distinguishing detail: "Oak sliding wardrobe, three doors, mirrored centre panel" |
Keyword stuffing in alt text is both an accessibility failure and a spam signal. If it would sound absurd read aloud, it is wrong.
Filenames and surrounding context
- IMG_4471.jpg tells a crawler nothing. oak-sliding-wardrobe-mirrored.jpg is free context
- Words separated by hyphens, lowercase, no spaces or underscores
- Nearby text matters: a caption and the paragraph beside the image both inform what it is understood to depict
- Store images in a sensible path — /images/projects/ beats /wp-content/uploads/2024/07/
Formats and sizes, in the order that saves the most
- Serve AVIF with a WebP fallback and a JPEG or PNG last. AVIF is typically 30–50% smaller than WebP at equal quality.
- Generate several widths and let srcset and sizes pick — one 2000px image sent to a 390px phone is the most common single performance mistake.
- Always set width and height attributes, or an aspect-ratio, so nothing shifts as images arrive.
- Lazy-load everything below the fold, and never the LCP image — lazy-loading your hero image directly delays your worst metric.
- Use SVG for logos, icons and line art. It is smaller, sharper and scales without variants.
Where image sitemaps are worth it
An image sitemap adds image locations to your XML sitemap so images that are hard to discover — loaded by script, or on pages that are rarely crawled — can still be found. It is worth doing when images are a genuine discovery channel: product photography, portfolios, galleries, before-and-after work.
It is not worth doing for stock imagery and decorative graphics, and it does nothing for images nobody would ever search for.
Social preview images
The Open Graph image is the one image most people will see, because it appears every time the URL is shared. It should be 1200 by 630 pixels, under about 300 KB — WhatsApp in particular often refuses to render larger files — and it should carry readable text, since it is usually viewed at thumbnail size.
A per-section image beats one generic card for the whole site: a link to a pricing page and a link to a case study should not preview identically. Generating them at build time from the page title keeps them consistent and stops them going stale.
Questions people ask about this
- Does alt text help rankings?
- It helps image search directly, and it contributes to what a page is understood to be about. Its primary purpose is accessibility, and writing it properly for that purpose produces the best version for search anyway.
- Should every image have alt text?
- Every image needs the attribute. Decorative images take an empty value so assistive technology skips them; omitting the attribute entirely makes a screen reader announce the filename, which is worse than nothing.
- Is AVIF safe to use?
- Yes, with a fallback. Every current major browser supports it, and the picture element handles older ones by serving WebP or JPEG instead.
Who wrote this
Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan — fifteen live client sites, six of them stores taking real payments. The examples in these guides come from those builds and from the audit that rebuilt this site, not from a content brief. More about how I work, or read the case studies.