You say that the sites claimed NOT to be pornographic. If they really can claim that then the police will surely be unable to take any action.
I think I would call them, however, and offer to identify to them the guy that was looking at the photos - more for their information than for any specific current action.
I would then ensure that there was increased visibility of the library rules regarding "acceptable use".
I would not ban the customer, but would keep a good eye on him in future & if it happened again then would tell him he was no longer welcome at the facility (what are the rules, can you actually ban someone & if so with what authority & under what rules?).
What I would NOT do is to inform anyone else about the customer's actions - I think that could be dangerous & would be unfair. These things can get well out of hand.