@Shinjidude: I'd say a combination of these things is...

Shinjidude
2009-12-03T05:03:37-05:00

I'd say a combination of these things is probably the best approach. First, start with a pool of well-known, active Contributors, then take a look at the quality (rather than the quantity) of their favorites and their uploads* (the random posts feature in their profile should be a good spot-check). If they pass muster that far and you still want to be sure they'll be acceptable, then offer those people the quiz. Those that are interested in the position and pass should serve as good Janitors.

*I think it's important that we look at both favorites and uploads because most people probably favorite only things of better quality than their average upload. If the person can identify the best images, but still uploads crap, they might not be the best person to pick which posts gets in and which don't.