code: function(oriImg,newImg,size){
          var width = size.width;
          var height = size.height;
          gm(oriImg).resize(width,height)
          .write(newImg,function(error){
              if(error){
                  gutil.log(gutil.colors.red(error));
              }
          })
      }
docs: [coverImage 转换图片方法]
  @param  {[string]} oriImg [原始图片path]
  @param  {[string]} newImg [新建图片path]
  @param  {[object]} size   [新图片的尺寸]
  @return {[void]}        [如生成失败，打印失败日志，否则无返回值]