Image Optimisation Checker
Check image formats, lazy loading and responsive sources across a page.
About this tool
Looks at how every image on a page is set up: its format, whether it lazy loads, whether responsive sources are offered, and whether the first image is being delayed by mistake.
That last one is the mistake worth catching. Lazy loading is applied site-wide by most tooling, including to the image at the top of the page — which delays the exact thing Largest Contentful Paint measures. The main image should load eagerly; everything below the fold should not.
This reads the markup rather than downloading the files, so it reports how images are configured, not how many kilobytes they are.
Common questions
Should every image lazy load?
No, and this is the most common mistake. Images visible when the page opens should load eagerly. Lazy loading the top image delays Largest Contentful Paint, which is the metric that image mostly determines.
Is WebP worth switching to?
Usually. It is typically 25 to 35 percent smaller than JPEG at equivalent quality and is supported everywhere that matters. AVIF is smaller still but slower to encode and less universally supported.
What does srcset actually do?
It offers the browser several sizes so a phone does not download a 2000-pixel-wide image to display it at 375. On image-heavy pages this saves more than changing format does.
Does this measure image file sizes?
No. It reads the HTML and does not download the images. Measuring real weight means fetching every file, which is a crawl rather than a request — this tells you whether the setup is right.