The Copy Your System Makes
You set the file. The desktop quietly saves a second version of it, and that is the one you look at.
Windows does not hang the file you chose. It transcodes it into a JPEG under AppData\Microsoft\Windows\Themes, at a quality of about 85 by default, and shows that copy instead. A registry value called JPEGImportQuality raises it to 100, which is the only part of this whole arrangement you get a vote in. macOS points at the original and leaves it alone; several Android skins resize first, then compress, and never mention either step.
Nothing you notice at first. The damage shows up where the picture changes slowly: a dusk sky that went from deep blue to pale in one smooth sweep now steps down in visible bands, and dark corners break into faint square blocks around eight pixels wide. Photographs full of grass, gravel or feathers hide it well, because detail gives the compressor somewhere to put its errors. Empty gradients have nowhere.
So the wallpapers that suffer most are the calm ones, which is unfortunate, because calm is usually why people pick them. If a sky develops stripes a week after you set it, the file on disk is probably fine and the second copy is not. Set the quality once, choose the picture again, and the screen goes back to being smooth.
