Something

#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;
}