Return a |PackURI| instance representing the next available image
        partname, by sequence number. *ext* is used as the extention on the
        returned partname.

def next_image_partname(self, ext):
        """
        Return a |PackURI| instance representing the next available image
        partname, by sequence number. *ext* is used as the extention on the
        returned partname.
        """
        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))