$(function(){

	var conf = {
	className : 'over',
	postfix : '_on'
	};
	$('.'+conf.className).each(function(){
	this.originalSrc = this.src;
	this.rolloverSrc = this.originalSrc.replace(/(\.gif|\.jpg|\.png)/, conf.postfix+"$1");
	preloadImage(this.rolloverSrc);
	}).hover(function(){
	this.src = this.rolloverSrc;
	},function(){
	this.src = this.originalSrc;
	});
  
});

preloadedImages = [];
function preloadImage(url){
	var p = preloadedImages;
	var l = p.length;
	p[l] = new Image();
	p[l].src = url;
}