How do you regain your bf trust after he found a pic of another guy dick in your phone?

that should worry him. that's hard to see that can make him think you are cheating or will cheat especially if he knows the guy. So the big question he is probably asking is why is it on your phone in the first place, you will have to answer that. but try to make it easy for him let him know you wont cheat on him that you don't like some1 else besides him. don't expect him to trust you over night give him time. I'm guessing the only big ways you could gain his trust back would be to ask him how talk to him i dont see him getting past it any other way.