My husband and I, including other family members post pictures of our kids on Facebook, but we make sure that the only people who can see them are genuine friends. I can understand how you frustrated you feel because you have no control over what she does.
Maybe you can work on her before you try the contacting FB angle? Show her articles or news items on how people can get information from FB and use it to their advantage (a friend of mine works in the NJ Police Department and he says "Online Forensics" cases show how a lot of crimes (yes, including those related to child abuse) are committed using information from social networks like FB, MySpace, etc.
Also, tell her she can post pictures of your children but she should limit these to only people she knows. There is a way to limit which friends see the photos (i.e., allow friends except name 1, name 2 to see photos).
From your side, reiterate to your children that they are, under no circumstances, to talk or walk away with a stranger, even if they use tactics like "I know your mom and dad" or "your grandma is hurt and needs you."
Finally, last resort but you need to talk to your husband about this, especially about the pic that has your son's name and school uniform on there. That is just carelessness on her part.