How to set a transparent style?

[<hunchback netcabo pt>]

Hi, 

i'm doing something that involves GtkDrawingArea, in which i paint
some GdkPixmaps, and some of this GdkPixmaps have some
transparent areas, the problem is when i paint them, the transparent 
areas are painted in white! How do i do so that the transparent areas aren't 
painted?

Thanks for any help,
Afonso