code: def ensure_image(name, url, images_root, md5, untar_to=None):
      
      image_dir_path = os.path.join(images_root, name)
      mkdir(image_dir_path)
      image_file_path = os.path.join(image_dir_path, os.path.basename(url))
      ensure_file(url, image_file_path, md5)
      if untar_to:
          prepare_image(image_file_path, untar_to)
docs: Ensure OS image at url has been downloaded and (optionally) unpacked.