Comment by 13of40

If this was the kind of hash where flipping one bit of the input completely scrambles the output, the bad guys would just flip one bit of the input to evade it. Obviously a PhotoDna type of hash is going to be be easier to cause a collision with because they're averaging out a ton of the input data. According to Wikipedia the classic way to do it is convert it to monochrome, divide it into a grid, and average the shade of each of the cells. If they're doing that you could probably just pass in that intermediate grid and it would "hash" to the same result as the original picture with no porn present.