What the checker measures in the photo and how
Product photo checker opens the photo in the browser and works on a reduced copy of its pixels. It takes the colour of a thin band along the four edges as the background and, if that band is plain, grows the background inwards until the colour changes sharply. What remains is the product. A transparent PNG is read from its transparency instead. The outline gives the fill on the longest axis and the edges the product touches.
The report belongs to one marketplace at a time. Each main-image rule of that marketplace that can be measured from a photo gets a line with the rule, the measured value and a status: Pass, Fail, Note or Not measured. A further line checks pixel size, file size and format where the marketplace sets them. A preview shows the photo with the measured outline, and choosing another marketplace grades the same photos again without loading them a second time.
When to use this checker and when a neighbouring tool
This is the tool for a main image that has to meet the rules of one sales channel. Among the free tools it is the one that separates the product from the background, so it is the one that reports fill, cut-off edges and shadows. Image size checker reads no colours: it takes pixel dimensions, file size and format and lists the result for every marketplace at once.
White background checker answers a narrower question in finer detail: it reads the pixels in a band along the edges and reports the share that is exactly pure white. AI image label checker reads the markers written inside the file and never the picture. When the photo itself has to change, Remove background can deliver the cut-out product on solid white, and Packshot studio is briefed to show the product alone on a white studio background.
How to read the report and what it cannot tell
One Fail makes the whole verdict a Fail. Not measured appears when the edges show a scene or a textured surface: without a plain background there is no outline, so fill, edges and shadow are left open instead of estimated. Note marks a rule that the marketplace words as a recommendation, which is never shown as a Fail, or a transparent cut-out tested against a white background rule, which still has to be exported on white.
The outline is a single box around everything that is not background. A prop, a hand, a hanger or a second item lies inside it and is measured as product, and a pale product on a pale ground can merge with the background. Text, logos, watermarks, borders and sharpness are not what the tool measures. A Pass covers the measurable main-image rules only; it is not the approval of a marketplace, and the channel's own rule page remains the last reference.