Comment 4 for bug 503061

For reference, eog opens the preferences window, in eog-window.c, around line 2827:

eog_window_set_wallpaper (EogWindow *window, const gchar *filename)
{
        EogWindowPrivate *priv = EOG_WINDOW_GET_PRIVATE (window);
        GdkScreen *screen;

        gconf_client_set_string (priv->client,
                                 EOG_CONF_DESKTOP_WALLPAPER,
                                 filename,
                                 NULL);

        screen = gtk_widget_get_screen (GTK_WIDGET (window));
        gdk_spawn_command_line_on_screen (screen,
                                          "gnome-appearance-properties"
                                          " --show-page=background",
                                          NULL);
}