var HelloWorldLayer = cc.Layer.extend({
          ctor:function () {

              this._super();
             var bg = new cc.Sprite(res.HelloWorld_png);
             var size = cc.director.getWinSize();//获取屏幕大小
             bg.x = size.width / 2;      //  x轴／2即为x轴中点
             bg.y = size.height / 2;    //   y轴／2即为y轴中点
             this.addChild(bg, 1);
             return true;
          }
});