I really relate with you on having my picture taken. I'm the same way, I don't honestly like my picture being taken and I usually shy away from it. It's just one of those things I'm not totally comfortable with due to trauma.

Your friends should be respectful and understanding towards your feelings and try not to push you. If it continues on happening you should talk to a close friend in the group about your feeling regardless of how you feel about the entire situation. I hope you resolve things and they understand your feelings.