//画像のロールオーバー
$(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;
});
});

//ロールオーバー向け画像プリロード
preloadImages = [];
function preloadImage(url){
var p = preloadImages;
var l = p.length;
p[l] = new Image();
p[l].src = url;
}