You can make it react more believably with just four image points (one in each corner) provided you calculate the force in a certain way..
First, instead of applying force to just the closest imagepoint, apply it to all four.
and secondly(you may have already done this), make the forces inversely proportional to the distance from the explosion...(so the imagepoints closest to the explosion have a stronger force applied to them)
It won't be perfect, but done this way, it will make objects rotate if the explosion is in the right place, like in your diagram.
you seem like you know what you're doing, but if you need more help i can send you a .capx i made a while ago with this in it.
a more 'realistic' way would be to make the explosion shoot out a bunch of smaller physics objects that collide with everything around it. (although that's overkill)