$(function(){
	var conf = {
		className : 'btn',
		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;
	});
});

preloadImages = [];	
function preloadImage(url){
	var p = preloadImages;
	var l = p.length;
	p[1] = new Image();
	p[1].src = url
}