function addEv(obj, type, func){
if(obj.addEventListener){ obj.addEventListener(type, func, false); }
else{ if(obj.attachEvent) obj.attachEvent('on' + type, func); }
}

var rolloverClassName = 'rollover';
var altKey = '_over';

var rolloverEvent = function(){
	
	var oImgs = document.getElementsByTagName('img');
	var oRolloverImgs = new Array();
	for(j = 0; j < oImgs.length; j++ ){
		if(oImgs[j].className == rolloverClassName){
			oRolloverImgs[oRolloverImgs.length] = oImgs[j];
		}
	}
	
	
	var preLoadImgs = new Array();
	for(j = 0; j < oRolloverImgs.length; j++){
		var src = oRolloverImgs[j].src;
		var pid = src.lastIndexOf('.');
		overSrc = src.substr(0,pid) + altKey + src.substr(pid);
		preLoadImgs[j] = new Image();
		preLoadImgs[j].src = overSrc;
		rollover(oRolloverImgs[j], overSrc, src);
	}
	
	function rollover(obj, over, out){
		obj.onmouseover = function(){this.src = over};
		obj.onmouseout  = function(){this.src = out};
	}

}

addEv(window,"load", rolloverEvent);