Someone is posting pictures of my child on the internet. How can I stop this?

You need to copy the web page or pages where the pictures are being posted, and you need to find out who is doing the posting, and who is hosting those web pages.  You need a lawyer, to file a lawsuit to get a court order called an injunction, ordering the pictures removed (and, later on, to try the case and get an award of damages for the invasion of privacy).

An injunction is something that takes special proof, that the only way to protect someone's rights is to order someone else to do (or stop doing) something.  It's even harder to get an injunction at the very beginning of a case (a temporary injunction or restraining order), but it may be possible here, because depending on the pictures, your lawyer might be able to convince a judge that each day the pictures remain posted causes you and your child to suffer "irreparable harm."