Just make up a number. You can use any number for your bluetooth passcode just so long as you both use the same one.
When your phone asks for a passcode just enter "1234" and make sure your friend does the same. Once you've both entered the same passcode, your two phones will be "paired." Once they are paired, set the phone as an authorized device so that you won't have to the passcode business everytime that you want to exchange pictures.
