conf = {
	className : 'Hover',
	postfix : '_over'
};
function setMouseOverImages() {
	$A(document.getElementsByClassName(conf.className)).each(function (node){
		node.onmouseout  = changeSrcFunction(node.src);
		node.onmouseover = changeSrcFunction(node.src.replace(/(\.gif|\.jpg|\.png)/, conf.postfix+"$1"));
	});
}
function changeSrcFunction(data){
	return function(){
		this.src = data;
	}
}
Event.observe( window , 'load' , setMouseOverImages , false );