Forcing desktop picture on clients

Is there an easy way to make user groups use a specific desktop pic when logging in? I'm at a small university and want a generic campus background for student sign-ons and don't feel like logging into all the accounts and changing them personally before locking them out of the preferences... I haven't really been able to find specifics on how to do this. I have the files locally on each client, but would be just as happy (or more so) having them on the server (OS X 10.5.7 and OD). I heard there was a UNIX command to set the desktop and could mass send that via ARD, but haven't been able to find any other info on that, either. Thanks.