Image SEO in 2026 boils down to three jobs: describe every image accurately, deliver it fast, and make it discoverable and attributable. This guide covers each with copy-ready HTML, sticking to what Google, web.dev and the browser makers actually document.
It starts with a rule that catches many modern sites: Google indexes images in an img element (including inside picture) but not images set as CSS backgrounds. Then description: alt text that says what the image shows and why it's there, in plain words, with empty alt for decoration and any text inside a graphic repeated in the alt. Filenames, captions, nearby text and page titles all help Google understand an image, and the title attribute isn't a substitute for alt.
For speed, I recommend serving AVIF or WebP through a picture element with a JPEG or PNG fallback (and making sure the server sends the right content type), setting width and height on every image to prevent layout shift, using srcset with sizes so phones don't download desktop files, lazy-loading below the fold, and never lazy-loading the main image, which should get fetchpriority="high" instead.
I also cover image sitemaps, the image property in structured data, licence metadata for Google's Licensable badge, and what's actually documented about Google Lens and multimodal AI Mode: use your own photos and explain them in text nearby. A checklist and honest limits close it out.
Key takeaways
- Use real img elements; CSS background images aren't indexed
- Write alt text that describes what the image shows
- Serve AVIF or WebP with a fallback
- Always set width and height to prevent layout shift
- Lazy-load below the fold, but prioritise the hero image
Related on BuiltToWinWeb: what makes a website load fast
Read the full guide on BuiltToWinWeb: The image SEO guide for 2026
A summary by Jacob Campbell of an article first published on built2winweb.com on October 7, 2026.