Wallpaper
Wallpaper (also desktop picture and desktop background) is an image used as a background of a graphical user interface on a computer screen or mobile communications device. On a computer it is usually for the desktop, while for a mobile phone it is usually the background for the ‘home’ or ‘idle’ screen. Though most devices comes with a default picture, the user can usually change it to a file of their choosing.
http://en.wikipedia.org/wiki/Computer_wallpaper