How do one put a shape upon a pixmap?

[Preben Randhol <randhol dusken4 samfundet ntnu no>]

(I'm wondering if this was asked earlier, but I cannot find it in the
mail archive. And I cannot remember the solution if any. Sorry)

I want to put a shape (a pixmap with a transparent colour) on top of a
pixmap. 

I'm trying to learn GTK and thus making a small game similar to
chess. I thought I wanted the squares in the board as pixmaps (as they
are not plain like chess) and the pawns as shapes on top of the
pixmaps. I have looked at the examples in the tutorial, but I cannot
figure out how to put the shape on top of the pixmap.

If somebody please has some hints on how to accomplish this I would be
most thankful.

-- 
Preben Randhol                    | Azbuka, to je obracenej nabytek.
Tlf    73940929/(735)94076 [arb]  |                  -- Bohumil Hrabal
Email  randhol@pvv.org            | [Transl:  Cyrilic, that's
http://www.pvv.org/~randhol/      |           furniture uspide down.]