$(function(){
    $("#menu ul li").hover(function(){
        $(this).children("a").children("img").attr("src",src(this,true));
    },function(){
        $(this).children("a").children("img").attr("src",src(this,false));
    });
});

function src(img, bool){
    var _img = $(img).children("a").children("img");
    var _src = $(_img).attr("src");
    var o = _src.split("/mnu")[0];
    var i = _src.split("/mnu")[1].split(".gif")[0].split("_")[1];
    var p = o + "/mnu_" + i;
    return (bool)? p + "_on.gif": p + ".gif";
}

function replace(str,ch,nw)
{
    return str.split(ch).join(nw);
}