To use this setting, type the fully qualified path and name of the file that stores the wallpaper image. You can type a local path, such as C:\Windows\web\wallpaper\home.jpg or a UNC path, such as \\Server\Share\Corp.jpg. If the specified file is not available when the user logs on, no wallpaper is displayed. Users cannot specify alternative wallpaper. You can also use this setting to specify that the wallpaper image be centered, tiled, or stretched. Users cannot change this specification. Note: You need to enable the Active Desktop to use this setting.
START > RUN > GPEDIT.MSC > USER CONFIGURATION > ADMINISTRATIVE TEMPLATES > DESKTOP > ACTIVR DESKTOP > FIND ACTIVE DESKTOP WALLPAPER > RIGHT CLICK PROPERTIES > CHECK ENABLED > FIND WALLPAPER > OK