function SimpleSwap(imgObj, which) {
    imgObj.src = imgObj.getAttribute(which || "origsrc");
}

function SimpleSwapSetup() {
    var x = document.getElementsByTagName("img");
    for (var i = 0; i < x.length; i++) {
        var oversrc = x[i].getAttribute("oversrc");
        if (!oversrc) continue;
        // preload image
        // comment the next two lines to disable image pre-loading
        x[i].oversrc_img = new Image();
        x[i].oversrc_img.src = oversrc;
        // set event handlers
        x[i].onmouseover = new Function("SimpleSwap(this,'oversrc');");
        x[i].onmouseout = new Function("SimpleSwap(this);");
        // save original src
        x[i].setAttribute("origsrc", x[i].src);
    }
}