eBay needs photos of at least 500 x 500 pixels and recommends about 1600 x 1600. Files can be up to 12 MB. This tool reads your photo and tells you where it stands on each number from eBay's help page.
eBay has no pure white rule, so the checker does not test your background. It runs in your browser and your photo stays on your device.
What eBay asks for beyond pixels
eBay suggests an uncluttered, neutral backdrop, with white generally best and a darker one for shiny items. The main photo should show the whole item face-on. Added badges, logos, copyright notices and watermarks can affect search placement, and used items cannot use catalog or stock photos.