I need some advice...

I'm going to apologize now because this is kind of long and drawn-out...
I've been with my boyfriend for about six months now. Neither one of us has had too many sexual partners and have been open about it if the other one has questioned. The last person he had sex with prior to me was someone who, according to him, is a friend of his and was about a year before we started dating. He says after that, they decided to just be friends; but knowing him and based on what he's told me about himself, I feel like what probably happened was he wanted to be more than friends and she told him she wanted didn't want to be more than that. He's still friends with her on FB and they follow each other on instagram and all that social media stuff, and I'm fine with that...doesn't bother me. The part that does get under my skin, and I don't know if I'm justified in this or not, is that he "likes" photos that she posts of herself from music festivals or going out or whatever...basically almost any pic where she's scantically dressed. So after some classic FB and instagram stalking, I've noticed he doesn't do this when other girl friends post pics of themselves. I don't know if this is something I should bring up to talk to him about or just let it go. I guess I'm worried he hasn't moved on from her, and I don't want to be hurt in the future if one day she decides she wants to be a part of his life, more than just friends. I need some advice on how to handle this or if there's even anything I need to handle. Also, I should mention that I'm a pretty sensative person when it comes to relationships, and can also be insecure about myself (developed from insecurities I had when I was younger)...so needless to say, this is driving me a little nuts and he can definitely sense something is bothering me. I just don't want him to think it's a jealousy thing because I don't feel like it is. And if I'm reading too much into this, I don't want him thinking that he needs to cut off all communication from her. Help!