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;
  };
}
};
//クラス名によるエレメントノード配列取得
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;
}
//プリロード
preloadedImages = [];
function preloadImage(url){
	var p = preloadedImages;
	var l = p.length;
	p[l] = new Image();
	p[l].src = url;
}