Select an affordable print that won't make a huge dent in your budget. This way, you won't be devastated if the humidity of the room eventually deteriorates the quality of the piece. Consider magazine pages, simple computer printouts or photocopies of your favorite pieces of art.
Select a plastic or metal frame for the picture. These materials stand up to bathroom moisture better than wood.
Replace the glass in the frame with plastic or acrylic. This will protect your bare feet from the danger of broken glass should the picture get knocked off the wall -- a serious concern in small bathrooms with little room to maneuver.
Choose a location for the picture that is as far from the shower as possible. This will reduce the damage caused by steam.