Grabbing a part of an image

I'm trying to allow the user choose part of an existing image and then copy that area to a separate bitmap file. The area may be any shape.

I see the clippath command for a canvas but I dont seem to see that you can do anything with what was clipped. It would be great if you could save that as a file or something.

Has anyone done something like this before and can give me an idea?

thanks.