What the checker reads from a file and what it lists
Image size checker takes three things from each file: its width and height in pixels, its file size and its format. The dimensions come from opening the image in the browser; the format comes from the file type the browser reports, or from the extension of the file name when that type is not a known one. No colours, no background and no product outline are read. Each dropped file gets its own card.
The card lists every marketplace for which the Herofab guides hold size or file rules, one line each, and the name on a line leads to that marketplace's guide. A line reads Passes, or it names each rule the file does not meet: a format that is not accepted, a file above the size limit, a side that is too short or too long, too many megapixels, an aspect ratio other than the required upright one, or dimensions other than the exact ones asked for.
When a size check is enough and when another tool fits
Use this checker when a finished image has to go to several sales channels and the question is which of them accept the file as it is. Product photo checker is the neighbour for a main image: it measures the background, the fill and the cut-off edges from the pixels for one chosen marketplace, and it adds that marketplace's size and format line to the same report.
White background checker reads the colour of the pixels along the edges of a photo, and AI image label checker reads the markers inside the file; neither compares the file with size rules. When a line here names a problem, a free tool can change the file: Marketplace image resizer fits the photo inside a marketplace's dimensions with padding and without cropping, Image compressor saves it as a JPG under a file-size limit, and HEIC to JPG converter turns a HEIC photo into a JPG.
How to read the list and what it leaves unmeasured
Each line compares one marketplace's limits with the file exactly as it is, and a megabyte is counted as a million bytes. A passing line can carry a note when the image is too small for zoom. If the browser cannot open a file, the card says that its dimensions could not be read and no rule is checked; a JPEG or PNG copy can be dropped instead. The first check runs without sign-up, and later checks continue with a free Herofab account.
Passes means that the file meets the size, file-size, format and ratio limits that the guide records for that marketplace, as checked on the date the page shows. It says nothing about the picture: background, fill, text and logos are not read. The format is taken from the type the browser reports or from the file name, not from the image data. A pass is not the approval of a marketplace, and the channel's own rule page is the last reference.