Next time it happens, I’d say, “Sorry, I’m just not comfortable being in the pictures. You guys go ahead without me. I’ll take the photo for you!” If they protest, hold firm, “It’s just how I feel about it guys, so sorry, please don’t make a fuss.” If they persist, then they are really callous and rude and I’d probably leave and stop going out with them. It sounds like they are all about posting the photos on social media. There always just has to be the group selfie! You have every right to your boundary on this, especially given your reason that the photo makes you feel critical of yourself. A friend will understand.