I personally see nothing wrong with naked baby pictures; my grandma has plenty of me in the bath in my baby albums. Child nudity, in my view, is only a problem when it's sexualised, and if someone sees a photo of a baby in a sexual light there is clearly something wrong with that particular viewer. That said, I think naked baby photos should be a private thing, only for close family and friends to see. Thus, posting them on facebook is probably not the best of ideas.

Regardless of my opinion, your family members are in the wrong because it's your baby and they MUST respect the mother's wishes. How you choose to raise your child, including what photos exist of him, are YOUR choice and no one else's, and it's really wrong of them not to respect your wishes on the matter. It seems rather disrespectful, to me. But considering what you've said about the family members in question, it seems telling them that would do little good; they may not listen to you anyway, unfortunately. But you've every right to be upset about it!

I'm pretty sure that if you report a photo on facebook, the poster of the photo won't find out who reported it; it'll just be taken down. Yes, I just double-checked with Facebook's Terms of Service, and reporting photos IS in fact completely anonymous. Maybe give that a try? And if it's not taken down or she just decides to repost it, report it again, I guess. Even if she thinks it might have been you, she'll have no proof; it could have been anyone on her friends list. Best of luck!