Replace links of placeholder images (the name of which starts with
        "py3o.") to point to a file saved the "Pictures" directory of the
        archive.

def __replace_image_links(self):
        """Replace links of placeholder images (the name of which starts with
        "py3o.") to point to a file saved the "Pictures" directory of the
        archive.
        """

        image_expr = "//draw:frame[starts-with(@draw:name, 'py3o.')]"

        for content_tree in self.content_trees:

            # Find draw:frame tags.
            for draw_frame in content_tree.xpath(
                image_expr,
                namespaces=self.namespaces
            ):
                # Find the identifier of the image (py3o.[identifier]).
                image_id = draw_frame.attrib[
                    '{%s}name' % self.namespaces['draw']
                ][5:]
                if image_id not in self.images:
                    if not self.ignore_undefined_variables:
                        raise TemplateException(
                            "Can't find data for the image named 'py3o.%s'; "
                            "make sure it has been added with the "
                            "set_image_path or set_image_data methods."
                            % image_id
                        )
                    else:
                        continue

                # Replace the xlink:href attribute of the image to point to
                # ours.
                image = draw_frame[0]
                image.attrib[
                    '{%s}href' % self.namespaces['xlink']
                ] = PY3O_IMAGE_PREFIX + image_id