What are my rights as a parent when it comes to photos of my child?

I found out yesterday that my ex's new gf put a photo of my child up as her profile picture on facebook as though he is her son! I've never met the girl, they haven't been together long and she met my son once (without my knowledge at the time & my ex tried to hide it). Surely someone must have permission to display photos of someone's child all over the internet! I have my pictures of my son set to private because I don't want every weirdo out there seeing them and she comes along and undermines me. Is there any legal advice about this?