On the one hand, I don't think it's unreasonable for family to want to have group photos taken. (I'd feel differently if someone was demanding multiple professional pictures of just you.) And as someone else said, if you're there for the wedding you'll probably be trying to look good anyway, and there will be so many other people in the photo that you'll kind of just blend in. Plus, I personally think no one is as critical of photos of me as I am myself (er, did that make sense?)--I mean, I've noticed that when I look at a photo of myself, all I see is the flaws, and other people like it. And the same with others--if my mom looks at a photo of herself, she'll immediately criticize it, while I think it looks fine.
All that being said, you are an adult and you simply don't have to be in the photo if you don't want to. I've always thought that was a nice thing about being an adult, that I can now choose to do this or that, or not, as long as I express my choice politely. I wouldn't talk about it at all beforehand, just smile and nod when the photo comes up. Find out the organizational plans--when the photo will be taken and where. Then, at the appointed time, vanish. Find a distant ladies' room, go out to your car, walk around the block, whatever works for you. If there's someone left behind that you can absolutely trust, have them text you when the photo is over. If not, I would give it a good half hour at least. Then come back and act normal. Determine what the likely reaction will be--will people chide you?--and prepare, perhaps with a good explanation (that might be a white lie).