window.onload = function(){
	var conf = {
		className : 'btn',
		postfix   : '_on'
	};
	var imgNodeList = getElementsbyClassName(conf.className);
	var node;
	for (var i=0, len=imgNodeList.length; i<len; i++) {
		node = imgNodeList[i];
		node.originalSrc = node.src;
		node.rolloverSrc = node.originalSrc.replace(/(\.gif|\.jpg|\.png)/,conf.postfix+"$1");
		preloadImage(node.rolloverSrc);
		node.onmouseover = function(){
			this.src = this.rolloverSrc;
		};
		node.onmouseout = function(){
			this.src = this.originalSrc;
		};
	}
};
function getElementsbyClassName(name){
	var elements = [];
	var allElements = document.getElementsByTagName('*');
	for (var i=0, len=allElements.length; i<len; i++) {
		if (allElements[i].className == name){
			elements.push(allElements[i]);
		}
	}
	return elements;
}
preloadedImages = [];
function preloadImage(url){
	var p = preloadedImages;
	var l = p.length;
	p[l]  = new Image();
	p[l].src = url;
}