How to stop being ashamed of being in photos

Hello everyone, I am a male (16 years old) that over the course of past year has lost 23 kilograms. Recently I went into high school and met new people from my class. Not everything is great but I have friends there. The thing is when they want to take a group photo I want to avoid it at any cost. And even if they manage to convince me to be in one I tell them to not post it anywhere (they are very active on social media like Instagram). When I look at that photo I still see the same fat guy I was a year ago. And because of that I do not know how to even pose in those photos. All of them make some sorts of signs (?) with their hands and it seems new to me. Thanks for any response