1. Open the unattend.xml file by opening it in Windows SIM from the OS Info tab of your deployment task sequence.
2. Add the Themes component found under Microsoft-Windows-Shell-Setup to the "7 oobeSystem" pass.
3. Configure the "DesktopBackground" property in the Themes component with the desired path, for example %WINDIR%\Web\Wallpaper\MyWallpaper.jpg.
Mitch Tulloch is a eight-time recipient of the Microsoft Most Valuable Professional (MVP) award and widely recognized expert on Windows administration, deployment and virtualization. For more tips by Mitch you can follow him on Twitter or friend him on Facebook.