Sorry, did not know you had found the cmd..
On another note: I recently had issue getting a group of users to display a separate background from the default domain set one, I had to get around it by forcing the background via a reg key import, however, since found out that the problem was caused by a setting in the Default User profile... once I replaced it with a clean/fresh copy, the GPO started working properly on all the machines in question... just a thought?