Right click the UltraMon icon in the taskbar notification area and go to "Wallpaper". You'll see a "Browse" button. Use it to go to the wallpaper you want to use. In the case of Win 7 backgrounds, you'd go to C:\Windows\Web\Wallpaper and then the subfolder containing the picture you want to use. Use the "Stretch" style if you want to use a single monitor wallpaper across multiple monitors.