Changing toolbar icons

[Jim Deas <jdeas jadsystems com>]

Hi,
 I am new to this so please no flames.
I am using Glade to create a user interface. Using
toolbars I have created a group of buttons for the
user. I would like to change to icon on the toolbar
in response to a user click. Using the snip below
I think I am getting the proper reference but how
do I change the displayed icon?
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void
on_button1_clicked                     (GtkButton       *button,
                                        gpointer         user_data)
{
//The button has been clicked.
GList* list = NULL;
GtkToolbar* toolbar;
struct GtkToolbarChild* child;
GtkWidget * tmp_toolbar_icon;

	toolbar = (GtkToolbar*)lookup_widget (GTK_WIDGET (button),
		"toolbar1");
	list = toolbar->children;//Get a list of all buttons here
	child = list->data;//get the first child on the toolbar
	tmp_toolbar_icon = gnome_stock_pixmap_widget (lookup_widget
		(GTK_WIDGET (button), "mainWin"), GNOME_STOCK_PIXMAP_OPEN);
	child->icon = tmp_toolbar_icon;*** This is wrong but why? ******
}
----------------------------------------------------------------------

Thanks for any help
JD