I would have called them.
I wouldn't have expected them to do much, if anything, but it would be up to them. There are loads of people out there looking at nasty pictures, and they might say their time's better spent going after people who are actually doing nasty things.

I would have called them, though, simply to cover my own butt.
(I probably would have called in advance just to ask what they thought I should do.) If someone pulls that computer for evidence, there's no difference between what the other guy had looked at and what I'd looked at except the reasons we both looked at it. It might be nice to know the cops are aware of your reasons before anything Pete Townsend-like can happen to you. Then again, talking to them might actually provoke them, in which case I'd make sure I had proof the library staff had asked me to look.