code: def next_image_partname(self, ext):
          
          def first_available_image_idx():
              image_idxs = sorted([
                  part.partname.idx for part in self.iter_parts()
                  if part.partname.startswith('/ppt/media/image')
                  and part.partname.idx is not None
              ])
              for i, image_idx in enumerate(image_idxs):
                  idx = i + 1
                  if idx < image_idx:
                      return idx
              return len(image_idxs)+1
  
          idx = first_available_image_idx()
          return PackURI('/ppt/media/image%d.%s' % (idx, ext))
docs: Return a |PackURI| instance representing the next available image
          partname, by sequence number. *ext* is used as the extention on the
          returned partname.