Is my Girlfriend right to be mad?

So me and my girlfriend are both 16, just finished our sophomore year. We've been dating for 6 months or so but have been friends for much longer. So one of my female friends is transferring to an east coast boarding school (we're in California) and was having a lunch and movie with a couple of male and female friends to kinda say goodbye. So I actually invited my girlfriend because I knew my friend wouldn't mind, but she was already doing something with some of her female friends.So she couldn't come, but I didn't mind in the slightest me going there. So there, my friend who is leaving, decided to take a bunch of group pictures and individual pictures of each of us to kind of have as a keep sake when she's gone. I felt like an asshole If I were to say no to a couple pictures, I've known her years and she might never see me again. So i was tagged in about 10 pictures, some group pictures, some of me and her, and thats when my girlfriend flipped. She feels its ridiculous to have "that many" pictures with another girl. She's not usually the type to get so mad at something like this, and she know I'm head over heals for her and would never do something with another girl or try to make her mad. I straight out owned up for what I did, told her the circumstances that she's leaving to the east coast, and gave a lengthy apology, but it wasn't enough to cool her jets. I don't know, I understand how she could feel a tiny bit uneasy, but I feel like she's overreacting and being a little immature about it. I also tried switching the roles and seeing how I would feel, but I personally trust her and know that she feels for me and not some random guy, I was hoping she felt the same way too. I know I wouldn't mind, especially if it was under the same circumstances that I was. When she first came to me about this we could only talk for a couple minutes as she's helping out at a small bakery, but I'm talking to her in an hour or so. I still plan on apologizing for this in hope to pacify her anger, but I was wondering, does she really have the right to be as mad as she is? Or did I actually slip up big time? Any tips for calming as well are appreciated.