What the checker looks at
Images are usually the heaviest part of a web page and the part most often left without text alternatives. This checker reads the page's HTML, lists every tag (including the real address behind common lazy-loading scripts that keep it in data-src), and then requests the first 64 KB of each image file. That is enough to read the file's format and its pixel size from the header, and the server's response tells us the full file size without downloading all of it.
For each image it reports what it found and what to change. Missing alt attributes and alt text that is just a file name are flagged, because search engines and screen readers rely on that text to understand the image. Missing width and height attributes are flagged because without them the browser cannot reserve space, so text jumps when the image arrives — a layout shift that counts against Cumulative Layout Shift.
- Alt text: missing, a file name, or longer than a sentence
- Width and height attributes, so the layout doesn't shift
- Lazy loading: missing on images further down the page, and wrongly applied to the first image
- Format: JPG, PNG, GIF or BMP over 30 KB with no WebP or AVIF alternative
- Weight: files over 200 KB, and over 1 MB as an error
- Pixel size: files far larger than the size the tag declares, or very wide images with no srcset
What it can't tell you
The checker sees the HTML and the image files, not the rendered page. It doesn't know which images appear on the first screen on a given device, so lazy-loading advice is phrased as a check: the first two images are never told to lazy-load, and a lazy first image is flagged because it is often the main image that decides Largest Contentful Paint. It also can't see images added by JavaScript after the page loads or set as CSS backgrounds.
Pages behind a login, pages that block automated requests, and addresses on private networks can't be checked. The tool fetches at most 3 MB of HTML and measures up to 40 image files per page, so very long pages are checked in part and the report says so.
Fixing what it finds
Most findings have a one-step fix. Heavy files can go through the image compressor, which usually cuts a photo to a fraction of its size without a visible difference. PNG and JPG files can be converted to WebP, which every current browser displays. Oversized images can be resized to the width they are actually shown at, and you can make two or three smaller copies for a srcset so phones download a smaller file.
Alt text and width/height attributes are edits to your HTML or CMS rather than to the file: describe what the image shows in a short sentence (or use an empty alt for purely decorative images), and add the image's pixel width and height to the tag. After uploading the fixed images, run the check again to confirm.