Sending Letters to Celebrities?

Okay, I'm a huge fan of the Harry Potter films. I REALLY REALLY want to send a letter to some of them (Probably Dan, Emma, Rupert, and Gary Oldman-he plays Sirius Black). Okay, so what I'm going to do is just write a short letter and ask for an autographed picture. So should I send in a picture of them, with an envelope ready for return, so they can just sign it? And if I do that, how big should the photo be? Is the 4" x 6" too small and dumb to use? should I print out bigger one? PLEASE DETAILS!