There is a common misconception that if pictures of a person were provided freely, that the recipient may do with them as he/she pleases. This is often not the case due to right to control one's own image known as the "right of publicity." And this right actually applies with more impact generally if the person is not a public figure. If the use of the photos is for commercial purposes (which it appears to be here), then the right is also often triggered. However, it does vary from state to state and you have the additional situation of having posted the subject photos to a web site that is presumably accessible worldwide. In short, what you are being accused of does, unfortunately, tread on real rights and you should consider trying to settle this amicably and/or seeing a local attorney if things escalate.
I hope this helps.
Disclaimer: This answer is for informational purposes only and
The right of publicity generally gives a person the right to control the commercial use of that person's identity (name, likeness, image, voice, etc.). "Commercial purposes" means that someone is making money from the photos. Even if you are not being paid, the fact that the website charges a fee for access means that someone is making money from displaying the pictures. (If it were a free site, the company operating it could still be generating revenue from selling ads on the web pages.)
Separately, there is the question of her right to privacy. If the photos were taken in a situation in which a person would have a reasonable expectation of privacy (as is presumably the case if she was nude), distributing or displaying them to anyone would likely violate her right to privacy under the laws of most states. Privacy rights can be infringed even by a non-commercial use. These laws vary from state to state, and it's often unclear which state's laws apply in the event of a dispute related to Internet content.
If you are facing a serious threat of litigation, I would encourage you to consult a defense lawyer in your state and avoid posting anything further on this site about the situation.