docs: def image(radar, at=None):
      '''Retrieve a radar image.
  
      :param radar: radar station no.
      :param at: stat datetime, defaults to now.
      '''
      at = round_to_5_minutes(at or datetime.utcnow())
  
      return ''.join([
          'http://image.nmc.cn/product',
          '/{0}'.format(at.year),
          '/{0}'.format(at.strftime('%Y%m')),
          '/{0}'.format(at.strftime('%Y%m%d')),
          '/RDCP/medium/SEVP_AOC_RDCP_SLDAS_EBREF_',
          '{0}_L88_PI_'.format(radar),
          '{0}00000.GIF'.format(at.strftime('%Y%m%d%H%M'))
      ])