If an ex sends you nude pictures is it illegal to post them online?

Legality may not be the right question. Frankly, unless the pictures are "obscene" it is probably not illegal. However, being legal does not mean that you might not have civil liability for invasion of privacy or other causes of action. Your facts do not explain why your ex would send you nude pictures but if they were sent with the intent that they be private then publicly posting them could cause you to get sued. Even if you ended up winning, you could end up losing in terms of time, effort, and fees to an attorney. Thus, if your desire is to punish by posting the pictures I would truly think twice about it and then think again. This is both legal and practical advice, which is why many attorneys refer to themselves as attorneys and counsellors at law.

Illegal - maybe. Dumb - absolutely. VERY DUMB. If the ex is a minor, posting or distributing those pictures would get you in serious trouble and ruin your life. Sexual predator type stuff. Lifelong registration. No one would hire you. Ever. Even if the ex is an adult, you could get sued at minimum under most any fact situation and you can be charged with a crime if it is alleged you took those pics without knowledge of the victim. Take the high road. If you were interested in someone at one time, why would you want to hurt them now (and in the process make yourself look bad for being vindictive and making a poor choice in getting involved with that person in the first place). Two wrong do not make a right. You will be respected more for not being a party to such nonsense and acting like a mature adult. Like my Grandfather used to say: Don't get down in the mud and wrestle with a PIG. After a while, no one will be able to tell you apart!