Image SEO: A Complete Playbook
File names, alt text, AVIF, lazy-loading, and the Core Web Vitals angle.
Images are 50% of bytes on a typical page. They are also a top source of CLS and LCP. Image SEO is performance SEO.
File names
Descriptive, hyphen-separated, keyword-aware. serp-preview-tool.webp, not IMG_4521.jpg.
Alt text
Describe the image for accessibility. Include keyword naturally. Don’t stuff. Don’t leave empty (unless purely decorative).
Format
- AVIF: Best compression. Slowest to encode. Supported by all major browsers.
- WebP: 30% smaller than JPEG. Universally supported.
- JPEG: Fallback.
- PNG: Only when transparency is required.
Sizing
Don’t serve a 2400px hero on a 400px slot. Use srcset with multiple sizes. Use sizes to tell the browser which one to pick.
Loading
- Above-the-fold hero:
fetchpriority="high", preload it. - Below-the-fold:
loading="lazy". - Decoding:
decoding="async"for non-blocking.
ImageObject schema
Required for image-rich results. Properties: contentUrl, thumbnailUrl, name, description, author, copyrightNotice, license.
Image sitemaps
For image-heavy sites, a dedicated image sitemap helps Google discover and index images faster.
The CLS angle
Always specify width and height attributes (or use aspect-ratio CSS). Reserve space. Don’t let images push content around as they load.