Originally posted by: SuperTool
It's so easy to plant this stuff on someone's computer, I am surprised this "evidence" stands up in court.
The government bears the burden of proving, beyond a reasonable doubt, that the defendant knowingly possessed the material. This is somewhat easier now that so many machines are on networks and require secure login - a forensic expert can easily testify when images were accessed and/or created with high specificity, and if that happened when the defendent was known to be logged in and working on other files, the government has a solid case in that respect.
In my experience, though, you're right that that tends to be a weakness of the government's case in these computer child porn prosecutions. Frequently the defendants are more tech-literate than the average bear as well, which is a further complication.