"As pointed out in post #56, computer forensics experts should be able to tell if this was planted."
Creating files on your computer and then a trail to make them look as though they've been there for years would be the work of but minutes for any skilled computer expert. I could probably do a pretty convincing job of it. Someone who's really into PC stuff could definitely do it.
ps Although I also detest kiddie pron as much as anyone, the mere existence of such pictures on a computer's hard drive should not be enough for a conviction, IMO. Additional evidence, for example a proven history of accessing sites containing such, or transmission of them to others, or membership in a site devoted to them should be required.