Once you've put a picture on Facebook, you have no control over what happens to it. All you need is one friend or family member to carelessly share it or copy it and suddenly it's out in the world.
How would you have felt, aged thirteen, if the school bullies had had access to really embarrassing photos of you? And used them to humiliate you?
How would you have felt if the boy you fancied and were just about to make headway with, was suddenly confronted with your embarrassing auntie tagging you in a "Look at you ten years ago!" photo? It may or may not kill his ardour, but it would certainly have killed thirteen-year-old me's confidence in pursuing the relationship.
You can't predict what sort of body image problems your teenager might have, and how those photographs will affect that.
You also can't predict what will happen to those photos. Mara Wilson, who played Matilda, discovered perfectly innocent pictures of herself on a foot fetish website when she was twelve. Strangers wanking off to her feet. She spent her entire adolescence hiding from cameras. Another mother had photographs of her Down Syndrome daughter stolen without her consent and used to advertise an early detection test which would help parents abort Downs foetuses. Exactly the opposite of the message her blog was trying to send.
You say you don't care that your mum posts pictures of you. Great. But many, many children and teenagers DO care, and you can't ask your three-year-old for informed consent until it's too late.