What Is Image SEO? More Than Just Alt Text

Jul 30, 2026 · SEO Glossary

Almost every performance audit I run tells the same story in the first five minutes: images are 60–75% of the page weight, and nobody has touched them. A photographer's portfolio site I crawled last spring served its hero image as a 6.4 MB PNG straight off the camera — scaled down to 1200 pixels wide by CSS, so visitors downloaded roughly eight times the pixels their screens ever displayed. That one file cost more load time than all the site's JavaScript combined.

What is image SEO? It's the set of practices that make your images fast to load, understandable to search engines, and eligible to rank in Google Images — covering compression, formats, dimensions, alt text, file names, lazy loading, and structured context. It lives at the intersection of accessibility, performance, and classic on-page SEO, which is why it gets dropped: no single team owns it.

What is image SEO built from? Start with weight

The performance half comes first because it's where the measurable damage lives. Images directly drive Largest Contentful Paint — on most pages, the LCP element is an image — making image work the highest-leverage input to Core Web Vitals on typical sites. Four practices cover most of it:

• Modern formats. WebP delivers the same visual quality as JPEG at roughly 25–35% smaller; AVIF pushes further still. Both enjoy universal modern browser support. If your build pipeline or CMS can convert automatically, this is nearly free savings across the whole site.
• Right-sized dimensions. Serve images at the size they display. The srcset attribute lets the browser pick the right resolution per device, so a phone doesn't download the desktop hero. My 6.4 MB portfolio example ended up at 140 KB — a 97% reduction with no visible difference.
• Sensible compression. Quality settings around 75–85 are visually indistinguishable from 100 for photographs at a fraction of the size. Export-at-maximum is a habit, not a requirement.
• Lazy loading — with one exception. loading="lazy" on below-the-fold images defers them until needed. The exception matters: never lazy-load your LCP image. Deferring the hero is a classic self-inflicted Core Web Vitals wound, and I find it on about a third of the sites that "optimized" their images with a plugin.

Alt text: written for people, read by robots

Alt text exists for screen reader users and for browsers that can't render the image. Google reads it too, which is why it became an SEO checkbox — and why so much of it is keyword-stuffed garbage that serves nobody.

The test that produces good alt text: describe the image to someone on the phone who can't see it. "Woman using a laptop" fails for a screenshot of a specific dashboard; "GA4 traffic acquisition report showing organic search sessions rising 40% over three months" succeeds. Keywords belong in alt text only when they genuinely describe the image. Decorative images — dividers, background flourishes — should carry an empty alt="" so screen readers skip them cleanly, which is different from omitting the attribute entirely.

In crawls of typical small-business sites I usually find 20–40% of images missing alt text entirely. It's among the most common findings in any audit report — partly because it accumulates invisibly: every content editor who drags an image into a page without filling the field adds to the pile.

The unglamorous details that add up

1. File names. blue-ceramic-vase-handmade.jpg gives Google a signal; IMG_20260214_093712.jpg gives it nothing. Rename before upload — renaming after publication changes URLs and needs redirects.
2. Surrounding context. Google infers image meaning heavily from nearby text, captions, and the page's topic. An image of a walnut dining table on a page about walnut dining tables ranks; the same image dumped in an unrelated gallery doesn't.
3. Explicit dimensions. width and height attributes let the browser reserve space before the image loads, preventing the layout jumps that Cumulative Layout Shift measures. Two attributes, one whole Core Web Vital defended.
4. Image sitemaps or structured data for sites where images are the product — e-commerce, recipes, real estate. For a standard business site, skip this and spend the time on compression.

Does Google Images traffic actually matter?

Depends entirely on what you sell. For e-commerce, recipes, travel, interiors, and anything visual, image search is a genuine acquisition channel — product images ranking well in Google Images put you in front of buyers who skipped the regular results. For a B2B consultancy, image traffic is mostly a curiosity. Be honest about which camp you're in before investing beyond the performance basics — the speed work pays for every site; the image-ranking work pays for visual businesses.

Finding your problems in one pass

Image issues are distributed — a missing alt here, an oversized PNG there — which makes them miserable to find manually and trivial to find with a crawler. A site audit lists oversized images, missing alt attributes, and broken image links across every page in one report, so you fix by template and by batch instead of by accident. On most sites, an afternoon of image work delivers a bigger speed improvement than a week of JavaScript tuning — it's the least glamorous high-leverage work in SEO.