Is it illegal to post a nude picture of someone on the internet without their promission?

It may be illegal (probably is) and is certainly the basis for a lawsuit. Difficult to imagine just how upset that might make someone. Seems very fraught with danger, besides the legal implications.

Barry is always so practical...take his post and really consider it. You are inviting a lawsuit. Though, it does depend on the ownership of the photo. If you are a porn site and the nude photo is of someone that was one of the models and they signed a consent, you will likely have no problems...best case scenario. If you were a peeping Tom and took photos of someone without their permission and no agreement of consent, you will likely have legal problems...worst case scenario. There is a whole grade of advice in between. Consult a lawyer in your jurisdiction. This is a big deal

She's not going to be a friend for long if you do it without her permission. This is a terrible idea. In many cases it may be inviting a lawsuit. Seriously, why would you want to do something potentially hurtful to another person, without their permission? Just don't do it.