Exactly! How on Earth could you prove an offending image wasn't yours?
In fact the case referenced in the article ran into this. The image was recovered from unassigned space on the drive and therefore there was not even any data about where it came from (that could be faked too but at least it gives you something to disprove).
Honestly, I've always been surprised that intelligence paychecks of all kinds aren't larger, for informants or agents. I know government salaries are generally constrained (especially for highly-skilled work), but surely one helpful step for fighting subversion is to insure that your employees are living comfortably?