Images are the largest thing on most web pages and the least examined.
A photograph gets exported at whatever size the camera produced, uploaded, and never thought about again.
That single unexamined decision affects three separate things at once: how fast your page loads, whether people using screen readers can understand it, and how well search engines interpret what the page is about.
Image SEO and accessibility are the same job approached from different directions, which is convenient, because doing one properly largely delivers the other.
Image SEO and Accessibility: Formats, Alt Text and Speed
Formats, briefly
- WebP offers substantially smaller files than JPEG or PNG at comparable quality and is supported across current browsers. It should be your default for photographs and most graphics.
- AVIF compresses further again, with support that is good but slightly less universal. Worth using with a fallback.
- SVG is the right choice for logos, icons and simple illustrations, since it scales to any size without additional files and is usually tiny.
- JPEG and PNG remain useful as fallbacks. PNG in particular is often used for photographs where it has no business being, producing files several times larger than necessary.
Size them for where they appear
This is the most common and most costly error on the web.
An image displayed at 800 pixels wide should not be a 4,000 pixel file scaled down by the browser. The visitor downloads all of it, and their device does the work of resizing it, for a result that looks identical.
Use responsive images so different devices receive appropriately sized versions. A phone should not be downloading a desktop hero image.
Add explicit width and height attributes. This lets the browser reserve the space before the image arrives, which prevents the content jumping around as things load. That jumping is measured as Cumulative Layout Shift and it is genuinely irritating to read through.
Lazy loading, with one exception
Images below the fold should load only when the visitor scrolls towards them. Most content management systems now do this by default.
The exception is your main above-the-fold image, which should load immediately. Lazy loading it delays the thing the page is judged on and makes your Largest Contentful Paint measurement worse.
Alt text, done properly
Alt text describes an image for people who cannot see it. That is its purpose, and every other benefit follows from doing it well.
- Describe what is there and why it matters in context. “Bricklayer applying mortar to a garden wall” tells somebody something. “Image1234” does not.
- Leave it empty for purely decorative images. An empty `alt=””` tells a screen reader to skip it. Omitting the attribute entirely causes the file name to be read aloud, which is worse than nothing.
- Do not stuff keywords. “Web design Gloucester web design agency Gloucestershire web design” is unusable for the person it exists to serve and is exactly the pattern search engines identify as manipulation.
- Skip “image of” and “photo of”. Screen readers already announce that it is an image.
If an image contains text that matters, put that text on the page as well. Text baked into an image is invisible to screen readers, to search engines and to anyone who wants to copy it.
The W3C’s images tutorial covers the decision tree for different image types thoroughly.
File names still matter
`bricklayer-garden-wall-gloucester.webp` describes its contents. `IMG_4471.jpg` does not.
It is a small signal rather than a decisive one, but it costs nothing to name files sensibly at the point of upload, and renaming them later is tedious.
The connection to everything else
Images typically dominate page weight, so correcting them is usually the largest single performance improvement available on a site that has never been reviewed.
Where performance and environmental impact both apply, treat them as the same lever: reducing unnecessary data transfer and processing supports a faster experience while lowering estimated digital carbon, though environmental figures remain modelled rather than directly measured.
The accessibility benefit is not a side effect. Alt text and correctly structured images are a requirement under WCAG 2.1 Level AA, and increasingly they appear in procurement questionnaires alongside everything else.
Our search engine optimisation work treats image handling as technical, accessibility and performance work simultaneously.