The 1st thing I have to ask is: do you routinely spy on your wife? If so, that's not to sign of a healthy relationship. How would you feel if she spies on your phone? Whether you would allow to do so not is not the point, you should respect her privacy.
My only problem with having compromising pictures on a mobile phone is what happens if it gets stolen? If she doesn't mind that her pictures may finish up being put on the Internet, like mine were a few years back (under different circumstances admittedly), then there is up to her. Though having said that, I think you are quite within your rights to tell her that you don't want her naked pictures splashed all over the place. There is the risk that she takes, every time she sets foot outside the door. Pickpockets and muggers regularly target mobile phones, and in your wife's case to thief would have an added bonus.
So, while I think you are out of order for spying on your wife, I have to agree that she is out of order for keeping naked pictures of herself on her phone when there is a very real risk of a phone being stolen at some point in the future, and then her pictures could finish up God knows where. If she likes taking naked pictures, put them on a CD on your PC, not on your hard drive. Once the photos of been transferred onto a CD-ROM, ask to delete them offer phone, and ask you not to take any new ones unless she saves them immediately onto a CD and then deletes them from a phone. She really should be thinking about her own safety.