Transparent background in window?

["Egon Andersen, Talura" <post talura dk>]

Hi,

Is it possible to make the background in a window transparent?
The reason that I ask is that I have made a splash screen with an image 
that is not rectangular (transparent background) and I would like the 
window to appear as having the same shape as the image.
I've been looking at gtk_style family, but I haven't found the solution.

Any hints?

Best regards
Egon Andersen