Save as an image content of a window

[José Antonio Caminero Granja <JCamGra teleline es>]

Hello once more,
        I want to know how to save as an image content of GtkWindow, because when I
use this I obtain a NULL image:

[...]
GnomeCanvas* canvas;
GdkImlibImage* image;
[...]
canvas = gnome_canvas_new ();
[...]   /* Putting items on canvas */
image = gdk_imlib_create_image_from_drawable (canvas->window, NULL, 0, 0, 300,
300)

Did I forget something?

Thanks a lot for helping me.

-- 
José Antonio Caminero Granja
Valladolid, España