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

If you had mentioned the age of your daughter one could have answered better. Assuming that you have daughter in her teens, I would suggest punishment would not be a solution. You must ask her or if not comfortable then tell your wife to do the questioning, as from where and why does she have such photos on her phone. Chances are that they are exchanges from her female friends. At this age, kids get exposed to such things and want to share with their friends to giggle over it or discuss it. Hope your daughter is just curious and she is just discovering this new thing she was always guarded off. Talk to her, and know her side of the story. You cannot stop your daughter from not seeing such photos or videos if she wants to, but the thing you can do is to, educate her about these things. Kids are vulnerable at this age and exposed to so many things that if we want them to be safe and not fall into any uncalled for situation, we should educate them. https://www.psychologytoday.com/blog/teen-angst/201308/teens-who-click-send-and-sext