Hi and welcome to the forum!
Ok, here is my "crash course"
Pictures: Most people have their own to send, but some celebrites need you to send one. As for buying vs printing, I go for printing my own (from DVDs or if I find one online I like). Some printing places (Wal Mart for example) are pretty picky about printing pictures of celebrities
Stamps: Most people to send a Sefle Addressed Stamped Envelope (SASE) with a request to a celebrity. Some people won't respond without it.
The only other tip I can think of off the rop of my head is put your address on the letter as well as the envelope addressed to the celeb--just in case they lose the envelope.
Good luck with your requests!
BossC