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