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;

          }

});