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.

def _next_image_partname(self, ext):
        """
        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.
        """
        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)