What to do when your boyfriend puts "I Love Yew" on pic of a girl you don't know?

Me and my bf had been dating for 2 years before we just broke up over this, but i was just wondering what other people would've done in this situation. My boyfriend does a lot of online chatting, which I don't like, but still allow because it showed that I was willing to trust him. But recently he's been chatting with this girl whom he listed as one of his "special someone's" along with me on his profile page. I decided not to worry about that because he always puts his friends in that box with me. But then I found a pic that he edited of the same girl, and he had put the words "I Love You" on the pic. I got really pissed and went off on him, asking if he was cheating on me because he had cheated on me in a similar way a year ago. But instead of answering me, he went off on me telling me that he was sick of being accused of things he hasn't done. He was sick of me not trusting him anymore because of what happened a year ago, and he broke up with me. My question is: What is your opinion of this complex situation? What would you have done when you found this pic? Do you think this is serious enough that we shouldn't get back together? If I wanted to fix our relationship, what's the best way to do that under these circumstances? PLZ HELP! :(