'Create a new standard image and return the newly-registered image-id
Note that the size of the new image is a random number between
1024 and 4096'

@classmethod
def _create_standard_image(cls, container_format, disk_format):

    size = random.randint(1024, 4096)
    image_file = six.BytesIO(data_utils.random_bytes(size))
    tags = [data_utils.rand_name('tag'), data_utils.rand_name('tag')]
    image = cls.create_image(container_format=container_format, disk_format=disk_format, visibility='private', tags=tags)
    cls.client.store_image_file(image['id'], data=image_file)
    cls.test_data = image
    return image['id']
