In Windows when you set a wallpaper it (sometimes) silently transcodes the one you selected into a new, smaller version. It doesn't always, there's a heuristic, but it's assumed it happens to prevent people from selecting a 1TB terapixel photo and have it destroy the machine.

Anyway, since it transcodes the WP into a JPEG, it has the ability to select a compression ratio. That ratio is pretty famously < 100% and as a result there's some degenerate cases where a wallpaper that looks good when viewed in the filesystem looks terrible when set to the background.

https://superuser.com/questions/1377883/how-to-prevent-wallp...