Oh I'm not saying that it wasn't worse for OP that she found pics of a XGF rather than a stack of mags, not at all, of course it's worse in that way.
Perhaps I'm wrong, i don't see those pics as porn, as they are probably not something he uses, as porn is often used, but is a souvenir.
So this is why I don't relate the pics with being porn. I could be wrong.
For me it's just terminology, of course she'd be devastated. doubly so when post natal hormones are in the fray too.
From what I read, the anger didn't seem to be at her reaction to the pics, it was that she read his diary.
I don't think there is anyone that realistically would be happy, OK, cool to find explicit pics of a former loved one.
They might be advanced enough to tell themselves that everyone has a past, but everyone of them would have preferred said memorabilia didn't exist. Double standards or no double standards.
SAB, know that this is no reflection on you, and you do need to discuss it with your DH, and I hope you can resolve it properly, with him binning the pics.
FWIW, it could be that he's hanging on to the stuff unconsciously to remind himself how far he's come. Perhaps the feelings he has for her today are more like, What an idiot I was for being involved with her to that extent'. He may feel anger toward her for his breakdown. Until you open your heart and tell him it'll be OK and allow him to talk about it, you won't know the truth and your head will make it all up.