Possible to Export a picture?

Well heres what I am trying to do, I pretty much trying to make a drawing program, I was just wonder, how can I make it so that when a person clicks on a button, its saves that file to their desktop as a .Bmp, what I am trying to make looks like this,

http://www.brightdev.com/robin105/Favorites/Sig%20Maker.html

I want it so that only the picture gets saved not the side buttons ;),

THANKS!!!