Image SEO: Alt Text & Compression (2026)

Images are an easy SEO win most sites ignore. Here's how to optimize alt text, file names, and image size for rankings, accessibility, and speed.

Images make up the majority of a typical page's download weight, yet image SEO is one of the most-skipped quick wins in the whole discipline. Done properly it pays off three ways at once: better accessibility for people using screen readers, faster page speed (and better Core Web Vitals), and extra traffic from Google Images. Here's the full playbook.

Alt text: describe, don't stuff

Alt text (the alt attribute) is a short written description of an image. Screen readers read it aloud, and search engines use it to understand what the image shows. Write a plain, accurate description of what's actually in the image; include a keyword only if it genuinely fits; and skip "image of" or "picture of" — the browser already knows it's an image.

<!-- Weak: vague and keyword-stuffed -->
<img src="1.jpg" alt="shoes shoes running shoes best shoes buy">

<!-- Good: describes the image, one natural keyword -->
<img src="blue-trail-running-shoes.jpg"
     alt="Blue trail running shoes on a rocky path">

Decorative images that add nothing to meaning (spacers, background flourishes) should use an empty alt="" so screen readers skip them. Leaving the attribute off entirely is worse than an empty one — some screen readers then read the file name aloud.

File names are a ranking signal

Rename IMG_4821.jpg to something descriptive like blue-trail-running-shoes.jpg before you upload it. Google reads the file name as a relevance hint for image search, and descriptive, hyphen-separated names are the standard. Rename before upload so you don't break links later.

Compression & modern formats (the speed win)

• Serve WebP or AVIF wherever you can. Both are dramatically smaller than the equivalent JPEG or PNG at the same visual quality — often 25–50% smaller — with wide browser support in 2026.
• Resize to the size you actually display. Don't ship a 4000px-wide original into a 400px slot and let the browser shrink it. You paid to download 100× the pixels the visitor sees.
• Compress. For photos, quality around 75–85% is usually indistinguishable from the original at a fraction of the size.
• Lazy-load below-the-fold images with loading="lazy" so they don't block the first paint. Do not lazy-load your main hero image — that one you want to load immediately (and ideally preload) because it's often your Largest Contentful Paint element.
• Always set width and height (or an aspect-ratio) so the browser reserves space and the layout doesn't jump as images load — that jump is Cumulative Layout Shift.

Before / after: one optimised image tag

<img src="blue-trail-running-shoes.webp"
     alt="Blue trail running shoes on a rocky path"
     width="800" height="600" loading="lazy">

How images tie into Core Web Vitals

Images touch two of the three Core Web Vitals directly: a heavy, uncompressed hero drags out LCP, and images without dimensions cause CLS. If images are your bottleneck, fixing them is the single highest-leverage speed change you can make — more on that in our guide to pass Core Web Vitals and the broader improve page speed playbook.

FAQ

Does alt text help rankings? Indirectly and honestly, yes — it helps Google understand the image (image-search traffic) and improves accessibility, which is a quality signal. It is not a place to stuff keywords.

WebP or AVIF? AVIF compresses slightly better; WebP has marginally broader tooling support. Either is a big win over JPEG/PNG. Many CMSs and CDNs can auto-convert.

Do I need alt text on every single image? Every meaningful image, yes. Purely decorative images should have empty alt="".

Find your missing alt text in seconds

The free Audit SEO tool flags images missing alt text on any page in about 30 seconds — the fastest way to catch the obvious gaps on an important page before you publish.