Surely, your company has a policy on this sort of thing?
What if you found a stash of crack in a customer's car?
Child porn is illegal, for one. You have become a witness to a (potential) crime. Best thing is to let the police handle it, to ensure that a) you don't get in trouble for "aiding" a criminal, and b) the "alleged perp" doesn't get off on some "invasion of privacy" technicality.
Take it to your boss, to ensure you follow company policy. It's the right thing and a legal obligation, IMO.
