Too early to disclose the specifics, but 2010 already has its first office prank underway. This one is a multi-day one (if I can make it all month without getting discovered, even better) and the victim is perplexed. Here's a hint: have friends join the prank so when you are the prime suspect suddenly the victim is hit by the friends in a way that makes it impossible for you to have done it (like you are in his presence when it happens). Good times.
(blogging about it may not be the best way to avoid getting caught, but if he is smart enough to search my blog for possible answers he would also be smart enough to check the printer logs to see who keeps printing those random pictures)