UBERent has recently announced a contest on their forums. All you have to do is give them an idea for a new wallpaper and if they like it, they'll make it into a reality, mail you a signed copy of the wallpaper, and a free copy of the game for you or your friends.
Link to Contest: http://www.uberent.com/forums/viewtopic.php?f=16&t=3169&p=34160#p34160
You have to sign up on the UBER Entertainment forums first.