It's not illegal to exchange pictures with another adult. It IS illegal to blackmail someone. So, if she tells you that she's going to post the pictures unless you pay her, then you can report her to the police. You're not the one who did something wrong in that scenario.
There are currently very few states that have criminal
laws preventing someone from posting nude pictures of another person on the internet. But it is a violation of privacy laws, and you could file a lawsuit if she released the pictures. In a situation where you have any reason to believe that she MIGHT release the pictures, then you have the ability to file a lawsuit and request an injunction against her.
Honestly, there is nothing you can do based on just a hunch that she might release the pictures. It's very easy to make yourself sick worrying about something like that, but there's no benefit. If she tries to blackmail you, call the police. The law is on your side. To take action, she either needs to do something with the pictures or give you an articulable reason to think she might.
If you have any questions or concerns about my response, please reply WITHOUT RATING. It's important that you are 100% satisfied with my courtesy and professionalism. Otherwise, please rate my service positively so I am paid for the time I spend answering questions. If you are on a mobile device, you may need to scroll to the right. There is no charge for follow-up questions. Thank you.