code: def _mark_image_file_deleted(cls, mapper, connection, target):
          
          cls._deleted_images.add((target, get_current_store()))
docs: When the session flushes, marks images as deleted.
          The files of this marked images will be actually deleted
          in the image storage when the ongoing transaction succeeds.
          If it fails the :attr:`_deleted_images` queue will be just
          empty.