code: function(css, imageIndex){
      var urlVal = css.value[imageIndex];
      var image  = this._isSpriteImage(urlVal, imageIndex, css);
      var images = this.images;
  
      if (image){
        image = path.relative('./', image);
        image = image.replace(/\\+/g, '/');
        images[css.id] = image;
      }
    }
docs: 获取css中背景图
  @param css {object} css规则
  @param imageIndex {number} 背景图属性所在的位置