window.onload = function(){
	var rootNode = document.getElementById("top_bt");
	var imgTag2 = rootNode.getElementsByTagName("img");
	for (var i=0; i<imgTag2.length; i++){
		imgTag2[i].onmouseover = function(){
			this.src = this.src.split(".png")[0]+"_e.png";
		}
		imgTag2[i].onmouseout = function(){
			this.src = this.src.split("_e.png")[0]+".png";
		}
	}
}
