How do I change my background back on Mac OS X?

One of my ';Mac-savvy'; friends decided to play a prank on me by using a Terminal command to change my background to a rather unpleasant picture.



Changing the background in my 'System Preferences' works for 2 minutes, after which the background he chose resets back to the unpleasant picture.



If any other ';Mac-Savvy'; people would care to help me out as to how I would go about reversing this, I'd be very grateful!How do I change my background back on Mac OS X?
http://thingsthatwork.net/index.php/2008



Then get him back:

http://lifehacker.com/373817/top-10-harm