Dynamically set image point

I'm trying to pick up an object with physics using the mouse.

I can do this easily but the object always moves from its default image point. If it's a long block I want to carry from where I clicked so the physics gravity rotates it and it "hangs" off the mouse.

Seems I need to dynamically set the image point of the block dynamically.

Any ideas how I do this?

Cheers

Toxic