I'm not sure how you would do it in group policy. Off the top of my head I don't think it would be possible, not really a target use for GP, though im not 100% sure and someone may know a way. The only way i can think of is create a user profile, set that as the background, then copy that profile into default user so any new users logging on the machine all get the mountain background. Any more than a few users to do this on would be quite tedious.