Are image-heavy pages using older formats?
It counts pages with several JPEG or PNG images lacking a modern source alternative.
What Check Engine measures
The test reads successful pages and selects image resources whose URL path ends in JPEG or PNG after the query string is removed. A saved modern-source flag excludes an image from the legacy set. At least 4 remaining images on a page are needed for that page to enter the finding. The count is then totalled across qualifying pages, rather than reporting every isolated older-format image. For example, 4 JPEG images with no recognised newer-format alternative can qualify a page for review.
The check reads image addresses and whether a newer-format alternative was found. It does not measure visual quality, transfer size or the browser's actual choice. Review the delivered picture before converting files.
How to read the result
The finding is low severity. A newer format alternative can satisfy the check while the delivered image still needs a quality or size review, and a valid older-format file is not automatically broken.
Why it matters
An image-heavy page offers a concrete place to inspect format choices. The check uses that concentration as a review cue, while leaving the actual tradeoff between quality, compatibility and file size to the owner.
How to fix it
Compare suitable modern encodings with the current images using the same visual content. Provide source alternatives where needed and keep a working fallback for your audience. Check the actual chosen source in a browser. Avoid replacing images blindly when a conversion loses useful detail or does not reduce the delivered file.
How to serve images in modern formats
What we found in field audits
Problems in Speed & Core Web Vitals were found on 13 of the 14 sites we audited. These are area totals. Read the field-audit method.
Method note
backend/seo_engine/checks/performance.py, modern_images, revision 0bb8e8d3.
Common questions
How many older-format images trigger the modern-image check?
The minimum is 4 remaining images after checking for newer alternatives. Fewer qualifying images do not reach the page finding.
Can a JPEG image with a newer source alternative be excluded?
Yes. A JPEG or PNG image can be left out when a modern alternative is recognised. Review the actual delivered picture separately.
Does the image-format check calculate the bytes a conversion would save?
No. Transfer size is not measured. Image URLs and newer-format alternatives supply the comparison.