You can place image points outside the image area. Only the origin and collision polys are bounded to the image area.
I can't recall off the top of my head why origins have to be bounded (I'm sure there was some reason), but it's very important for collision polys: a really huge optimisation in the collision tester is that it can immediately return with a "not overlapping" result if the object bounding boxes do not overlap, which is an almost instant test. If collision polys can go outside the bounding box, all bets are off and every single collision poly point has to be tested. That would be really slow, so collision polys are bounded to the object area.
Collision poly and image points are stored in texture co-ordinates - in other words, if the image size changes, all points and the poly scale accordingly. You probably want to make sure you crop before setting the collision poly, otherwise the collision poly will be scaled down accordingly too.