Here's the steps:
- copy the wallpaper image to a network share that all users have read access
- right-click the Default Domain Policy and select Edit
- expand User Configuration\Policies\Administrative Templates\Desktop\Desktop
- in the right-hand pane, double click Desktop Wallpaper
- select Enable and then type in the path to the location of the wallpaper
When the users logon again they will have the new wallpaper.