You said DH supports you in this. If both of the parents have made this decision, or agreed on the decision, then it's rude for others to ignore it. Even if DH doesn't feel as strongly about the picture issue as you do, he respects your desire not to have pictures of Baby on the internet.
I would sit down and talk to him about it first of all. Explain to him why it's a big deal to you. Help him understand 100 percent why you feel so strongly about this. Hopefully, even if he doesn't agree, he will understand that this is very important to you. The two of you need to talk and come up with an understanding.
Once the understanding has been met, then talk to family. If you guys both agree that a few pictures that are not tagged are OK, then let family members know not to tag her. Whatever the decision is, make it clear that the decision was made by BOTH of you.
If you see people posting pictures you aren't OK with, ask them politely to take them down and explain why. If issues arise, let facebook remove the picture.
As for MIL...that sounds like a whole nother pot of nonsense that DH needs to deal with.