>Should I fess up, or pretend I never saw anything?

Pretending you never saw anything would make sense - if you could 100% never hold it against her.

But given the reaction that you already described, the next best thing would be to tell her that you saw some pictures - and while you were kind of surprised (and a little confused). But no harm no foul. IF that's something you can honestly do.

While if you're going to hold it against her for as long as you're together - then she deserves to hear the truth - so she can decide whether or not you're worth putting up with.