Oh Yuck I'd hate to be in your situation. He sounds like he really genuinely cares for you but it comes down to the simple fact that you've talked to him, told him how it bugged you and he still didn't do anything about it to make you feel more comfortable. This may be me getting feminine but I think it's really disrespectful of him to leave pictures up around the house of them. It seems like hes not completely over her. Because when my ex and I broke up after 3 years I got rid of everything that wasn't "too special" and put it in the closet.
I think you need to give him an ultimatum. It's ridiculous to even have to worry over something like this but he obviously isn't very sensitive to how you're feeling about it or he'd take it down. Like you said you can't look at your future with him if hte past is always slapping you in the face(that was cute by the way).
Talk to him and say you're not comfortable with the pictures of them together all over the house. Say you should have pictures of us in their place. Never settle for less hun, everyone has to realize their worth more than they give themselves credit for.
I think you're being totally reasonable about this too. I dont even think it's a jealousy thing with you, it just bugs you which is totally understandable.
You may love him but there's no point being unhappy with someone. If he really cares for you like he says he should have no problem taking them down. Just give him the ultimatum.
One day the Moon said to me, "If he makes you cry, why dont you leave him?" I looked up at the Moon and said, "Moon would you ever leave your sky?"