First of all, this is a great question. Lots of people have to deal with this now…and it’s a great topic for discussion.
I don’t have a pat answer….but I don’t like being photographed. Once, I was followed around a shop by a person with a movie camera. I asked them to stop…and they didn’t. I felt really harrassed and left the shop because the management didn’t have an answer as to what to do.
I think that people now think that because they have phone cameras they can take photos of everyone. I had my photo taken once by a relative (because I didn’t say “No” and I should have) and it ended up on some slideshow for another relative’s funeral. I really found that appalling. I didn’t want to be up there at someone’s funeral or any other function. It really bothered me. And no, I am not on Facebook. A friend of mine who is in a highly professional position was horrified to find a photo of himself taken thirty years ago, when he was in college posing with a giant prop bottle of Jack Daniels at a frat party, looking thoroughly sloshed. The picture had been posted on the website of one of his younger cousins who had found the photo in a family album and thought it would be a scream to post it. (Uh, it was a scream, but not that kind.)
I think you should just say “no.”
Honestly, I wouldn’t even “pose” for a photo…I would take your relative aside and just say, “I don’t mind being photographed for the official family photo…but otherwise, would you mind not snapping my pix? Thank you for respecting my privacy.”