Load the image part collection with all the image parts in package.

def _gather_image_parts(self):
        """Load the image part collection with all the image parts in package."""
        for rel in self.iter_rels():
            if rel.is_external:
                continue
            if rel.reltype != RT.IMAGE:
                continue
            if rel.target_part in self.image_parts:
                continue
            self.image_parts.append(rel.target_part)