code: function (size, background, foreground, format, text) {
      // Random.image( size, background, foreground, text )
      if (arguments.length === 4) {
        text = format
        format = undefined
      }
      // Random.image( size, background, text )
      if (arguments.length === 3) {
        text = foreground
        foreground = undefined
      }
      // Random.image()
      if (!size) size = this.pick(this._adSize)
  
      if (background && ~background.indexOf('#')) background = background.slice(1)
      if (foreground && ~foreground.indexOf('#')) foreground = foreground.slice(1)
  
      // http://dummyimage.com/600x400/cc00cc/470047.png&text=hello
      return 'http://dummyimage.com/' + size +
              (background ? '/' + background : '') +
              (foreground ? '/' + foreground : '') +
              (format ? '.' + format : '') +
              (text ? '&text=' + text : '')
    }
docs: /*
  生成一个随机的图片地址。
  
  替代图片源
  http://fpoimg.com/
  参考自
  http://rensanning.iteye.com/blog/1933310
  http://code.tutsplus.com/articles/the-top-8-placeholders-for-web-designers--net-19485