Mine

############# PASCAL VARIANT, ONLY WORKING ON LINUX #############

program gtk_icon;

uses glib2, gtk2, gdk2, gdk2pixbuf;

var window: pGtkWidget;
var icon: pGdkPixbuf;

begin
	gtk_init(@argc, @argv);
	
	window := gtk_window_new(GTK_WINDOW_TOPLEVEL);
	// need to download file from http://www.penma.de/guckets-icon.png
	icon := gdk_pixbuf_new_from_file('guckets-icon.png', nil);
	
	gtk_window_set_icon(GTK_WINDOW(window), icon);
	
	gtk_widget_show(window);
	
	gtk_main();
end.

############# C++ VARIANT, WORKING FOR tml ON GimpNet WITH MINGW #############

#include <gtk/gtk.h>

int main( int   argc,
          char *argv[] )
{
    GtkWidget *window;
    
    gtk_init (&argc, &argv);
    
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    // need to download file from http://www.penma.de/guckets-icon.png
    gtk_window_set_icon_from_file(GTK_WINDOW(window), "guckets-icon.png", NULL);
    gtk_widget_show  (window);
    
    gtk_main ();
    
    return 0;
}