code: def _next_image_partname(self, ext):
          
          def image_partname(n):
              return PackURI('/word/media/image%d.%s' % (n, ext))
          used_numbers = [image_part.partname.idx for image_part in self]
          for n in range(1, len(self)+1):
              if n not in used_numbers:
                  return image_partname(n)
          return image_partname(len(self)+1)
docs: The next available image partname, starting from
          ``/word/media/image1.{ext}`` where unused numbers are reused. The
          partname is unique by number, without regard to the extension. *ext*
          does not include the leading period.