08-01-2012, 01:01 PM
If you want to reduce the JPEG artifacts, you can manuall save the image as a low compression JPEG (99-100%) to "%AppData%\Microsoft\Windows\Themes" (just copy into the address bar of any explorer window), then right-click it and "set as wallpaper". If Windows compresses it, quality drops drastically because some people at Microsoft thought noone had the storage space for high quality pictures (the lack of PNG support is just ridiculous).