These illegal photos are not trivial to obtain. Possessing (and here, the printing step necessitates possession) these illegal photos is in and of itself a crime in most relevant jurisdictions.

But OK, let's say that you've found a way to get the photos and you're comfortable with the criminal implications of that. At that point why don't you just hide the printed photos in your coworker's desk? My point is that if you have a disgruntled coworker who's willing to resort to heinous crimes in order to screw you over, there's many different things they could do that are less convoluted.