'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):

    def image_partname(n):
        return PackURI((u'/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))
