This is suspicious, but it's not enough to condemn the man just yet. I gather the tapes were vanilla, given that you didn't elaborate, but the photos appear incriminating in context, as they were found together with the tapes.

My question is why would your dad put a bunch of casual holiday pictures together with the inappropriate photo, if he used this photo as porn? If he intentionally mixed those casual pics in to make the album seem innocent, wouldn't it have been easier to keep it away from the porn in the first place? Like, in a drawer or something? I mean, leaving pics of your naked daughter next to "Backdoor Sluts XXL" is some next level kind of idiocy if you really mean to hide sexual attraction to your daughter.

It is of course possible that something bad is going on, but it might also be that either your dad or your mom stashed it in there at some point without thinking over how it would look. I think something perverse would have been better covered up, if it's first attempted.