Who had installed the cam? Probably some little goon from Magnar or even himself. There is not much behind that, you only need opportunity, and there were plenty. (and those cams cost not much either, a food-bill at the cafeteria is more expensive than some little basic spycam)
As for the camera, we know that some of the photos sent to the Jocks come from this camera (the angle fits perfectly). So the nerds have access either to the camera or to its images.
On the other hand, I'm amazed that a nerd could have put it there.
The location is not insignificant, since it's the spot used by the DIKs to get laid at parties. It's known only to them.
To install it, you have to climb a ladder, dismantle the lamp, put the camera on and put everything back in place. All relatively quickly, because the only time a stranger can do it is during a party.
Finally, there's the fact that it flashes after the power goes out. If it's powered by the lamp's electrical circuit, this makes it even more difficult to install it quickly and discreetly in the middle of a party.
So I think the camera was installed by a DIK. Either Vinny last year (DIKs refer to the fact that it would be like him to want videos of his sexual exploits), or Nick, because he's a voyeur. If Nick doesn't remember what he was doing with the computer, it's just because he was checking out the camera footage, which allowed the hacker to discover this in addition to posting Katy's photo.
) at least there should be a faded out choice somewhere, like DIK/NEUTRAL/CHICK system so ppl can figure it out w/o having to deep dive through the 'lore' like fanatics. 
