What to do? My boyfriend was sent a pic of some girl's boobs a few months ago but it's still on my mind. I'm not sure if he's friends with this girl or not, but I've been cheated on in the past and I do not want it to happen again.
"... A few months ago ..."
Since you don't mention anything happening since then ... You either trust him or you don't. If you don't trust him, you shouldn't be his girlfriend.