Is it against the law to post nude pictures of someone without their permission?

First of all, you need to hire a good lawyer. There are two levels for that lawyer to consider: Criminal and Civil. You stated he is your "ex." I will assume that you mean ex-spouse or ex-boyfriend. If this man threatens to post nude pictures and videos of you to harass you, he may be violating the Court's Protective Order, and could be jailed for being in Contempt of Court. Further, the act of posting nude pictures/videos online in order to harass you may also constitute a civil tort, which may also give you the right to sue him for economic damages and a civil injunction. However, that is more complicated. You obviously need to hire a good lawyer.

The first question to ask is ownership of the photos and videos, NOT posting online. After you decide the ownership issue, then look at the other issues.