bitmap - picturebox - cut, copy, paste irregular region

i'm writing a paint (xp) style application + need help - code, ideas, anything appreciated.

i can't work out a way to cut or copy + then paste an irregular part of an image. i know how to select an area then copy it to a correctly shaped irregular picturebox floating above the original. so far no problem. the problem is retrieving it from the clipboard it'll be returned as a rectangular bitmap.

does anyone know of a way to edit out the parts of the rectangular bitmap that i don't want + paste the (irregular) bitmap into a correctly shaped picturebox?

does that make sense?