Images are usually the heaviest thing on a page and the most neglected for SEO. Done right, they earn traffic from Google Images, help accessibility, and keep your Core Web Vitals green. Done wrong, they're the reason your LCP is slow and your alt-text column is empty.
Alt text
The alt attribute describes an image for people who can't see it — screen-reader users and search engines alike. Good alt text is specific and concise:
<!-- Bad -->
<img src="chart.png" alt="chart">
<!-- Good -->
<img src="chart.png" alt="Bar chart showing 40% traffic growth in Q2 2026">Describe what the image shows, don't keyword-stuff, and skip "image of" — screen readers already announce it as an image. For purely decorative images, use an empty alt="" so assistive tech skips them.
File names
A file named DSC_00421.jpg tells Google nothing. blue-running-shoes.jpg gives it context for free. Use lowercase, hyphen-separated, descriptive names — it's a small signal, but it's effortless.
Modern formats: WebP and AVIF
If you're still shipping JPEG and PNG everywhere, you're sending files 25–50% larger than they need to be. WebP is universally supported now and a safe default. AVIF compresses even better but encodes slower — great for hero images you can pre-process. Serve them with a fallback so old browsers still get something:
<picture>
<source srcset="hero.avif" type="image/avif">
<source srcset="hero.webp" type="image/webp">
<img src="hero.jpg" alt="..." width="1200" height="630">
</picture>Dimensions and lazy loading
Two attributes prevent most image-related performance problems. Always set width and height (or an aspect-ratio) so the browser reserves space and doesn't shift the layout — that's your CLS score. And add loading="lazy" to below-the-fold images so they don't compete with your hero for bandwidth. Critically, do not lazy-load your LCP image — that delays the one image that matters most.
Image sitemaps
For image-heavy sites (portfolios, stores, publishers), listing images in your sitemap helps Google discover them for Google Images. For most sites, well-marked-up inline images in the page are enough. See robots.txt and sitemaps for the fundamentals.
Auditing your images
The two questions worth auditing on every page: which images are missing alt text, and which are oversized? SEO Inspector lists every image on the page with its alt text (flagging the empty ones) and its dimensions, so you can spot a missing description or a 2MB hero in seconds — instead of discovering it in a slow Lighthouse run three weeks later.
