Masturbating to pictures of people you know is a big No-No. It happens without their consent (they probably don’t want this) and it’s not very healthy for yourself. If I’d do it, I’d struggle looking them in the eyes after that. I’d just stop and find some other fantasy.

Edit: And concerning your question with the celebrities… At least they get something out of it. It might happen without consent as well, but they get a lot of money for that, which your friends don’t.