Him hooking up with her was no accident. You two have been having problems, so he started looking around, and he met her during that time, and that's why he wanted to take a break from you. It was all part of the plan.
This dude is very untrustworthy, definitely not someone who I would say is relationship material. If I were you, I would dump this guy. He has shown you is true colors.
You say you "cannot get the image (of him and another chick) out of your head", and that's a very understandable way to feel, especially since this is so fresh.
If you do decide to give him another chance, I personally would not but I am not you, then that is your decision, but if find that after a time what he did still haunts you, then that's when you know it's time to move on, because otherwise you're staying in a situation where you will never have any peace.
Remember this: "Sometimes it is better to leave a bad situation, than to stay and try to fix it."