My Boyfriends Coworker Sent Him Questionable Photo

My boyfriend works with this girl, let's call her P. About a year ago, I was somewhat suspicious and went through his text messages, low and behold, I found an exchange between him and P making plans to hang out. He made plans with her, but never followed through. He didn't tell me about it, and when I confronted him he acted guilty. Basically, I decided to get over it since he, a) didn't cheat and b) I felt I was being overbearing.

Long story short, due to an upheaval at his work he told me she wasn't working there anymore (some staff got let go, etc). On our shared computer, I went to hotmail to check my account. He hadn't signed out of his email account and the first email in his list was one from here. She had sent him a photo of herself in her halloween costume, which wasn't exactly revealing but it wasn't modest either. It turns out that she is still working there, and he had basically lied to me when he said she wasn't.

I don't want to be a controlling, jealous, girlfriend, but I feel like he is hiding something from me. I'm starting to feel uncomfortable because i'm becoming this person that I don't want to be by snooping through his emails and finding stuff that upsets me. When I confront him about it, I don't really get straight answers and he gets defensive. I feel like I deserve better. My standard in relationships when it comes to friendships is that you should be open and communicative about who you're hanging out with. I don't want to hide things or have things hidden from me.

Should I break up with him? Yes or no?