To support the tree view, Gtk_Cell_Renderer_Pixbuf also supports rendering two alternative pixbufs, when the is-expander property is TRUE. If the is-expanded property is TRUE and the pixbuf-expander-open property is set to a pixbuf, it renders that pixbuf, if the is-expanded property is FALSE and the pixbuf-expander-closed property is set to a pixbuf, it renders that one.