You have to make a LogonScript that sets the WallPaper at every new user logon, because the wallpaper will not change until the user boots after 1. logon.
Script is VBscript:
Run at 1. logon (administrator):
'»»» Sets Wallpaper to None
WshShell.RegWrite "HKEY_USERS\.Default\Control Panel\Desktop\Wallpaper", "", "REG_SZ"
Run at 1. logon (user):
'»»» Sets Wallpaper
WshShell.RegWrite "HKEY_CURRENT_USER\Control Panel\Desktop\Wallpaper", "Path\bmp", "REG_SZ"
'»»» Sets Wallpaper to Center
WshShell.RegWrite "HKEY_CURRENT_USER\Control Panel\Desktop\WallpaperStyle", "0", "REG_SZ"