I am deeply sorry to know of your loss. This is not something you need to deal with today, or tomorrow, or the next day. Put them away and face the issue when you are ready. Only you can make that call.
As for the pictures themselves, I would be mindful that there was a lot more to your mother’s relationship with this woman, than just illicit intimacies. I have somewhat unwittingly found myself becoming kind of an apologist for gay people lately — not sure how that happened, I guess it’s just a function of trying to treat all people with kindness and decency. Telling people that sin is not sin — as many sinners, both gay and straight, practically beg us to do (so they will have some kind of affirmation of their desires and lifestyle) — is neither kind nor decent. But there is more to two gay people loving each other, than just sex and sexual attraction.
Just as we do not tell married people, when their marriages are declared null, that their relationship with that putative spouse was garbage and to be consigned to the dustbin, and just as we do not tell divorced and invalidly “remarried” people that their relationships are garbage and to be consigned to the dustbin, so neither should we treat gay people that way. If I had a gay couple living next door to me (there was a gay bachelor who lived with his mother, but no couples, not yet anyway), I wouldn’t shun or avoid them. I would be as decent and civil to them, as I would be to anyone else. And as I said elsewhere on CAF recently, nobody ever had an abortion as a result of a gay liaison. Nobody ever used an abortifacient contraceptive as part of engaging in a gay liaison. Gay sex is mortally sinful, but a lot of things people do are mortally sinful. And there shouldn’t be any need (and may not be any need) for me to say this, but I’m not gay. Contented bachelor, yes. Gay, no. But even if I were, I would be just as precious in the Sight of God, and would deserve nobody’s opprobrium. I’m celibate now, and l like women. I’d be equally celibate if I liked men.
I say keep the pictures and cherish them.