Append a ``<p:pic>`` shape to the group/shapetree having properties
        as specified in call.

def add_pic(self, id_, name, desc, rId, x, y, cx, cy):
        """
        Append a ``<p:pic>`` shape to the group/shapetree having properties
        as specified in call.
        """
        pic = CT_Picture.new_pic(id_, name, desc, rId, x, y, cx, cy)
        self.insert_element_before(pic, 'p:extLst')
        return pic