Well first off, masturbation is a very natural thing, and it’s nothing to be ashamed of. As far as using your friends photos, if you feel incredibly ashamed you could try stopping, but many people masturbate to people they find attractive or have romantic or sexual feelings towards. Dating is a complicated thing, but I recommend taking that step. Try talking to someone you may be interested in, and just go from there! Invite them for coffee, or to a movie, and just hangout and get to know each other first.