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