I'm LOVING this. I'm sorry!

Only a kind-heart reader here. I am 31 and my husband has quite a nice seat too!

To address your question, yes: I think you are quite adorably over-reacting. I think you should take his "sharing" of the photo more of a personal compliment. I would not be surprised if he wanted all his friends to know that YOU like his toosh ENOUGH to take pictures of it. You just made your man feel like a trophy and I find it neat that he knew where the pictures were - my husband has no idea where any of the pictures of him I take are. I suppose they aren't as good as yours!
Bless your soul. Be proud he loves the pictures enough to share, and sound your trumpet to all the ladies that you have something wonderful: him.
As far as his casualness about sharing his bare hiney: I doubt he viewed it
like that when he was flipping through them. Us ladies fore-think everything. Men are more "free fliers" who don't put weight into every little thing. If my husband shared that type of image...I'd smile, give him the eye, and tell him he needs spanking.