Return an |ImagePart| instance newly created from image and appended
        to the collection.

def _add_image_part(self, image):
        """
        Return an |ImagePart| instance newly created from image and appended
        to the collection.
        """
        partname = self._next_image_partname(image.ext)
        image_part = ImagePart.from_image(image, partname)
        self.append(image_part)
        return image_part