Pics, what do they mean????

Me and my boyfriend where going through our phones together, laughing at embarrassing old pictures and deleting stuff to clear up storage in our phones. Then I see sexy pics of another girl, I know this girl it is a friend of his and the dates on the pics shows he got them before we started dating. He said that she sent him those to ask if another guy would like them and he could keep them if he wanted, I know this is true because she told me the story of doing this when she was drunk. However, what has me worried is not that he has them (I forget about some of the stuff in my phone) but that he just scrolled past them and didn't delete them like the other "unimportant" pics. He saw I was uncomfortable and said he will delete them. My problem is that he didn't delete them right away and probably wouldn't have if I wasn't there. Am I overreacting? What would you do and what do you think? Why was going to keep them if I didn't say anything????