code: def _images_failed(cls, session, previous_transaction):
          
          for image, store in cls._stored_images:
              store.delete(image)
          cls._stored_images.clear()
          cls._deleted_images.clear()
docs: Deletes the files of :attr:`_stored_images` back and clears
          the :attr:`_stored_images` and :attr:`_deleted_images` set
          when the ongoing transaction has done rollback.