I noticed That if I download a wallpaper by my mobile and then I set it everything is fine. But if download it by my Computer and then copy it to my phone it's still fine but when I set it to my background it'll be a really bad quality. It's the same when I use a wallpaper application. The pictures are all good resolution (in the gallery) ~2000x3000px. I don't know what causes that strange bug.
