I'm still suspecting missing dithering to be the problem.
The function gdk_draw_pixbuf (http://library.gnome.org/devel/gdk/stable/gdk-Drawing-Primitives.html#gdk-draw-pixbuf) explicitly allows you to set a dither mode (http://library.gnome.org/devel/gdk/stable/gdk-GdkRGB.html#GdkRgbDither) but I'm not a GTK coder, so I can't test that...
I'm still suspecting missing dithering to be the problem.
The function gdk_draw_pixbuf (http:// library. gnome.org/ devel/gdk/ stable/ gdk-Drawing- Primitives. html#gdk- draw-pixbuf) explicitly allows you to set a dither mode (http:// library. gnome.org/ devel/gdk/ stable/ gdk-GdkRGB. html#GdkRgbDith er) but I'm not a GTK coder, so I can't test that...