You have a right to opt out of photographs. Clearly, it makes you very uncomfortable. You don't owe it to anyone to pose for a picture. Stick to your guns, and let your "No" be "No." Stick with that consistently! Sometimes, you kinda have to train people. I also agree with the suggestion that you talk with your friends individually about how you feel and would appreciate some understanding.

You don't really need to understand why they're not more respectful. They don't really need to know why you don't like being in photos. Any reason you give is apt to seem foolish to them. You simply have the right to decline. Believe that yourself, and keep stepping away from in front of the camera. Decide that this is not a negotiable matter. Hopefully, you won't have to give up your friends over this matter.

If this is just one of many issues that these friends and you have trouble working out, then maybe you would do well to cultivate other friends. If this group keeps pushing and pushing, then they strike me as mean people.