Here's the clincher:
"The Web site had about 250,000 subscribers. Holmes said authorities tracked down some of them using electronic and credit card information gathered in an investigation conducted by the postal inspection service, U.S. Customs Service, the FBI and the Dallas Police Department. It is illegal to possess child pornography."
Implication: It is NOT a legal defense to say "I didn't know the pic I posted was of a minor." Anything less than 18 years of age may result in fucked up shit like what happened above.