Whitelists are probably a lot more practical. With a blacklist there is the problem that vast numbers of new files will be coming on the network all the time,

Whitelists are probably a lot more practical. With a blacklist there is the problem that vast numbers of new files will be coming on the network all the time, and so you would need some mechanism to determine which ones should be put on the list. There are only two ways to do that, neither practical. One is machines, but then you need a huge server farm and lots of sophisticated software, and even then you get lots of false positives and false negatives. The other is human beings, but that would require a huge army of people. You can't get that many volunteers as people don't want to spend their time, and besides they don't want to be traumatized by looking nasty pics. So you have to hire them, and where would the money come from? As a consequence, I think white lists are the way to go.