Am I over reacting?

My bfs phone went off to be repaired so he's borrowed my spare phone for the past few weeks, his phone got delivered yesterday and he transferred everything from my phone back to his. This morning I sent him a sexy snap and he told me to save it and put it in the album he has with just my pics and to get the ones on messenger and also put them in the folder too which I done but while on messenger I noticed there was a chat thread with his ex gf who he was with for 18yrs and has 2 children with, I got told she had a 2 year affair behind his back and that's why they split, anyway it was a photo that had been sent last on the thread so curiosity got the better of me and I opened it, it was a naked pic of her, I scrolled and scrolled and the thread was full of them, this thread was before me and him got together and that doesn't bother me what does bother me is why he's still got that chat thread with her pics on, was full of sexting too, doesn't hurt me what's on the thread as we weren't together but hurts me that he still has them and can look at them whenever he pleases, he's told me to get over it which has just made me more angry and I've told him I need a break so he's gone to stay at his mums for a few nights while I 'get over it'

Edit....me amd him have been together 14 months now