Amusing anecdote: There was a story about this issue on Slashdot one time, where someone possessing kiddy porn had obscured the faces by doing a swirl distortion, but investigators were able to sufficiently reverse this by doing an opposite swirl and so were able to identify the victims.

Then someone posted a comment to say that if you ever want to avoid this problem, you need to do something like a Gaussian blur, which deletes the information contained in that portion of the image.

Somebody replied to that comment and said, "Yeah. Or, you know, you could just not molest children."