Main image background must be pure RGB 255,255,255. Off-white, eggshell, or light gray fails the automated check even though it looks white to the eye.
Walmart Seller Center requires product images that are publicly reachable, sized within the platform's bounds, in supported formats (JPEG/PNG typically), and free of overlays, watermarks, retailer logos, or text callouts. This issue trips when one of those constraints is broken.