Using open source to fight porn

Our daughter was rummaging through a box of memorabilia and found an evelope of photos taken in early 2001, about the time I'd purchased a cool new macro lens. One minute she was flipping through a series of cute puppy pictures and the next minute she's face to face with a set of full-frontal nude photographs depicting...a wolf spider. In fact, the spider was so exposed, the close-up so extreme, that Amy could not bring herself to even handle the photos so as to put them back into the envelope from which they came.

So when I got home I did the manly thing and, judging their scientific value to be near zero, tossed the spider pictures into the trash. When our daughter threw something away later that evening, and then needed to retrieve it, she shrieked again at the vile images that she could not unsee (and I was chastised for merely disposing of them instead of using our new commercial-grade shredder). Is there any possible way to prevent our children from accessing images or content that is disturbing to them or to us as parents? And should that be our sole criteria for judging whether or not we have won the war on porn?