$(function ()
{
    var IMAGE_LENGTH = 9;
    var num = (Math.floor(Math.random() * IMAGE_LENGTH) + 1).toString();
    num = num.length == 1 ? '0' + num : num;
    $("#mainbg_01").attr("src", "images/top_image/main"+num+"_01.jpg");
    $("#mainbg_02").attr("background", "images/top_image/main"+num+"_02.jpg");
});