Request the photo download url with the photo id
    :param photo_id: The photo id of flickr
    :type photo_id: str
    :return: Photo download url
    :rtype: str

def get_photo_url(photo_id):
    """Request the photo download url with the photo id
    :param photo_id: The photo id of flickr
    :type photo_id: str
    :return: Photo download url
    :rtype: str
    """
    args = _get_request_args(
        'flickr.photos.getSizes',
        photo_id=photo_id
    )
    resp = requests.post(API_URL, data=args)
    resp_json = json.loads(resp.text.encode('utf-8'))
    logger.debug(json.dumps(resp_json, indent=2))
    size_list = resp_json['sizes']['size']
    size_list_len = len(size_list)
    global image_size_mode
    image_size_mode = size_list_len if size_list_len < image_size_mode \
        else image_size_mode
    download_url = resp_json['sizes']['size'][-image_size_mode]['source']
    return download_url