Here is a (20M) guys perspective.

If he is dating you, it is because he wants to be with you. Just bc he has pics of other girls doesn't mean anything. You can't control him that much, what he chooses to do in his private time is on him. You should be more understanding and stop invading his personal space. Guys jerk off, if you think he is thinking of you every time he jerks off then I have bad news for you. This is completely normal, every man does this whether his girl knows or not. As long as he doesn't act on it then you have nothing to be mad about. Even the way you phrase it, "he confessed saying he used them to masturbate" ... is wrong. Stop looking at it like he "confessed" and start looking at it as "he was honest and told the truth". This is really something so minor that it is not worth your time causing problems over, appreciate his honesty and move past it.