I found sexually explicit photos on my daughter's iPhone; should I punish her?

I'm assuming your daughter is probably in the pre-teens or early teens, as that is the age when most children start getting curious about sexuality. If you ask me, I'd say hold on to the 'punishment' thing. Instead of reprimanding her, try to talk to her about why she has these images on her phone. It is possible that she has the pictures out of curiosity and has already discussed it with her friends. While it is obviously bothering you, for her and her friends, it may be something to laugh over and then forget. This is also a grave security concern, and you should educate your daughter about the many dangers that can happen in the virtual world. For all you know, someone shared these images and your daughter or her friends may be thinking of sending their own. Talk to her openly instead of being angry, so that next time, she comes to you directly for any concern, instead of hiding it.