官网没有具体例子，只有看api，研究成果如下

        var that = this;
        var url = "http://xxxxxx";
        cc.loader.loadImg(url, null, function(err,img){
        	var logo  = new cc.Sprite(img); 
        	that.addChild(logo);
        	logo.x = size.width / 2;
        	logo.y = size.height / 2;
        });

cocos2d js 如何动态加载外部图片