gistfile1.txt

function init()
{
var alt = new Array()
    ,banner = new Array()
    ,EXTENSION
    ,htmlImg = {}
    ,i
    ,idx
    ,myImg = {}
    ,PATH
    ,str
    ,url = new Array();
PATH = "images/";
EXTENSION = ".png";
banner = [
    "test01"
    ,"test02"
    ,"test03"
    ,"test04"
    ,"test05"
    ,"test06"
    ,"test07"
    ,"test08"
    ,"test09"
    ,"test10"
];
alt = [
    "test01"
    ,"test02"
    ,"test03"
    ,"test04"
    ,"test05"
    ,"test06"
    ,"test07"
    ,"test08"
    ,"test09"
    ,"test10"
];
url = [
    "test01.html"
    ,"test02.html"
    ,"test03.html"
    ,"test04.html"
    ,"test05.html"
    ,"test06.html"
    ,"test07.html"
    ,"test08.html"
    ,"test09.html"
    ,"test10.html"
];
setTimeout("init()",4000);
for(i = 0; i < 3; i++)
{
    idx = Math.floor(Math.random() * banner.length);
    myImg[i] = new Image();
    myImg[i].src = PATH + banner[idx] + "_on" + EXTENSION;
    htmlImg[i] = document.getElementById("bn" + i);
    htmlImg[i].setAttribute("src", PATH + banner[idx] + EXTENSION);
    htmlImg[i].setAttribute("alt", alt[idx]);
//★ここから、No.2 の回答にしたがって...
//  htmlImg[i].setAttribute("onmouseout", "this.src='" + PATH + banner[idx] + EXTENSION + "';");
//  htmlImg[i].setAttribute("onmouseover", "this.src='" + myImg[i].src + "';");
    htmlImg[i].onmouseout = (function() {
                                var s = PATH + banner[idx] + EXTENSION;
                                return function() {
                                        this.src = s;
                                    }
                            })();
    htmlImg[i].onmouseover = (function() {
                                var s = myImg[i].src;
                                return function() {
                                        this.src = s;
                                    }
                            })();
//★ここまで
    htmlImg[i].parentNode.setAttribute("href", url[idx]);
    delete banner[idx];
    str = banner.join(",");
    str = str.replace(/,,/, ",");
    str = str.replace(/^,/, "");
    str = str.replace(/,$/, "");
    if(i != 2)
    banner = str.split(",");
    delete alt[idx];
    str = alt.join(",");
    str = str.replace(/,,/, ",");
    str = str.replace(/^,/, "");
    str = str.replace(/,$/, "");
    if(i != 2)
    alt = str.split(",");
    delete url[idx];
    str = url.join(",");
    str = str.replace(/,,/, ",");
    str = str.replace(/^,/, "");
    str = str.replace(/,$/, "");
    if(i != 2)
    url = str.split(",");
    }
}
try
{
    window.addEventListener("load", init, false);
}
catch(e)
{
    window.attachEvent("onload", init);
}