I usually don't go out of my way to explain privacy to people, but I have to explain to almost every new acquaintance that I don't have to have a particular reason to refuse being in photos. One of the most egregious cases went in the line of "I hung out with that kind of famous guy once and he asked me not to post pictures of him, I can understand that, but why would you, a nobody, refuse being in pictures!?"

Saddest part is that even people who agree to not take pictures of you without further explanaion will usually throw quips at you every time there is a camera nearby.