Is it legal to post nude pictures of my ex girlfriend online or via txt without her consent when she personally sent them to me?

It is probably technically legal, as long as she was over the age of 18 when the photos were taken and she consented to you taking and using the photos. However, it is immature and ethically questionable. And even if it is legal, it could start a whole bunch of issues for you that you might have to deal with, including investigations by state or federal agencies, civil claims, etc.

Yes it is illegal. Although she is of age to consent, she did not consent to public display/exposure/dissemination/publication of her pictures. She might have a case against you. She can try to sue you for defamation and various invasion of her privacy, etc. since she only consented to you seeing the picture NOT the public. Whether she wins or not depends on the facts and if she can prove the elements. You might want to consult with an lawyer if she wines up trying to sue you... You should take it down immediately to mitigate the possible harm done to her reputation...etc.

Extremely bad idea.

I agree with my learned colleagues. It may or may not be legal, but in any context this a direct request for trouble. If you posted pictures of my 19 year old daughter on the internet with, but especially without consent would have to decide what I would do as a lawyer I can several things I could and would do, as a father, brother, or new boyfriend I am thinking "hospital job" or something equally distasteful for you my friend and I would be sure you would never see it coming. Any any light I can't imagine why you would do this, other than she dumped you. Legal or illegal it's trouble for you. Find a new girl. Take the higher path son. Put it this way, I don't think you'll need a lawyer unless you do post the pictures. Good Luck.