Not necessarily. But it means he still has feelings for her. Or at the very least, he wants to sleep with her again. As relationship red flags go, this is a pretty big one.
This isn’t like finding porn, which, despite the staggering number of questions we get about it, wouldn’t be all that big of a deal. This is someone he cheated on you with. At the very least, it’s beyond disrespectful to keep nude photos of her around. Even a clothed photo of her would be unfair and douche-y of him to keep around.
Confront him. This may end the relationship, but you can’t let it go. It’s a breach of trust and no one deserves that. Maybe they’re old and he forgot about them. But probably not. There is little grey area here. I am not
sure what happened with him cheating, but you must have found a way to
forgive him. Even if he’s not cheating on you with her now, he’s keeping evidence of his past indiscretion as jerk-off material. And that’s more than a little scumbag-y.