code: def get_or_add_image_part(self, image_file):
          
          image = Image.from_file(image_file)
          image_part = self._find_by_sha1(image.sha1)
          if image_part is None:
              image_part = ImagePart.new(self._package, image)
          return image_part
docs: Return an |ImagePart| object containing the image in *image_file*,
          which is either a path to an image file or a file-like object
          containing an image. If an image part containing this same image
          already exists, that instance is returned, otherwise a new image part
          is created.