image optimization
Also called: image SEO
Image optimization is the practice of compressing, sizing, naming, and tagging images so they load fast and can appear in image search. It spans file format (WebP or AVIF), file weight, descriptive filenames and alt text, responsive srcset markup, and lazy-loading images below the fold.
Two jobs: speed and discovery
Every image does double duty. For performance, the file has to be small and correctly sized so it does not drag down Largest Contentful Paint, one of Google’s Core Web Vitals. web.dev’s guidance is blunt: serve modern formats like WebP or AVIF, compress, use responsive srcset so phones do not download desktop-sized files, and never lazy-load the LCP image, since that always adds load delay. Lazy loading is for below-the-fold images only.
For discovery, the image needs signals Google can read. Google’s image SEO docs call alt text “the most important attribute” for image metadata: write a short, descriptive alt such as Dalmatian puppy playing fetch, not stuffed keywords. Filenames matter too (my-new-black-kitten.jpg beats IMG_001.JPG), as does placing the image near relevant text on a topical page. Use standard <img> tags, because Google does not index CSS background images, and consider an image sitemap for files a crawler might miss.
<img src="black-kitten.webp"
srcset="black-kitten-480.webp 480w, black-kitten-960.webp 960w"
alt="black kitten sleeping in a wicker basket"
loading="lazy" width="960" height="640">Skip loading="lazy" on your hero image and set fetchpriority="high" instead, so the LCP element is not delayed.
How it affects your traffic
Image optimization moves two levers at once. Compressed, right-sized images lift Core Web Vitals scores, which feeds page experience and keeps mobile visitors from bouncing before content paints. Descriptive alt text and filenames open a second channel through Google Images, which sends referral clicks that never touch a text SERP. In on-page work, image cleanup (format conversion, alt text, and set width/height to stop layout shift) is one of the cheaper wins because it touches every page at once.
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