I obvioulsy haven't seen the pictures but I would imagine they are just a picture of a baby in a bathtub...which I wouldn't view as a big deal but thats me. Unless the pictures are vulgar, which I seriously doubt they are, there is no way this is a legal issue. Like a couple people said maybe myspace can do something about it but I wouldn't hold my breath. You can go to court and see what they will do but that will take forever.
What you should do is go over or call the house and calmly explain your objections to your ex and his girlfriend. If you all act like civilized adults then you will be able to work out a comprimise, if not then go to court.
I don't know the situation here but is it because you think it's an inappropriate picture or is it because you don't like your ex's gf having pictures of your child on her myspace? I would guess it's a little bit of both. If the pictures were of him in a fully clothed outfit playing would it still bother you? Perhaps that can be the comprimise. If the thought of her having your child on her page, of the thought of her somehow replacing you or becoming a second mommy is the issue, then that's all on you and you have to get over that. Sooner or later you both will have other husbands, wives, or serious gf's/bf's, you should be glad that at least this gf seems to have an apprciation for your son which basically means better care for him while he is visting his father
On the other hand naked pictures always come back to haunt, christ I remember one with me and my girl cousin as 1-2 year olds in the same bathtub....ugghhh still makes me cringe....