Would you be upset?

I was looking thru my boyfriends pictures on his phone.. as he does it to mine all the time. We dont have anything to hide. As I was looking thru his phone there was pictures of him and another girl. The time frame of it was before we got together. Then I saw old pictures of them kissing and all their conversations. This all was right before we got together. I asked and he just said they were friends. I didn't think friends kissed each other. I'm upset he still has those pictures when we are together.. would you be upset or am I overreacting?