Sharing pictures may seem private. But in reality, nothing digital is ever 100% guaranteed private, you can't control where it ends up. The best way to avoid any problems is not to take the picture in the first place.
The problem with the Internet is that once information is ‘out there’ in cyberspace, it is out there forever. There have been numerous stories around the world regarding teens who have become victims of blackmail or bullying due to nude images of themselves circulating online. What may have started as a private gesture from a young girl to her boyfriend could turn into a nightmare if (or when) the photos fall into the wrong hands. There have been stories of young girls who have committed suicide because nude photos have circulated amongst their classmates or schoolmates. Unfortunately, once these images have been sent over the Internet it is virtually impossible to retrieve them. What may have started as a lark may end up haunting a young woman or young man for the rest of their lives.
It's natural to want to make your partner happy, so it may seem hard to say no to a request like this. But here's the thing: If your partner is a good, s/he will have more respect for you when you stay true to your values. Saying no is a chance to help your relationship grow by letting your partner know more about what you believe and stand for. You are better off safe than sorry.