Where a seller does not upload listings by hand, a feed carries them: one entry per product with its title, price, stock and one or more image links. The marketplace fetches each image from the address in the feed, so the image has to be online, reachable without a login and served over a secure connection.
Feeds usually separate a main image from additional ones, each in a field of its own, and platforms apply their image rules to both. An image that breaks a rule is refused during import or, on some price comparison sites, costs the offer its placement.
Many platforms fetch an image once and keep their copy. Replacing the file at the same address then changes nothing on the listing; the feed has to carry a new address before the image is imported again. Giving a changed image a new file name is the simple way to make that happen.