It sounds like you are referring to a screensaver, not wallpaper. Wallpaper is the an image in the background of the desktop. A screensaver runs when the computer is not being used; you can set how long it waits before the screensaver starts or you can disable it completely. Also note, if the computer sleeps or hibernates that may happen before the screensaver ever even starts up, so you may have to adjust those times as well if you want to see the screensaver.