var preloadFlag = false;
function preloadImages() {
    if (!document.images) return;
    on = newImage("images/on.gif");
    preloadFlag = true;
}

function newImage(arg) {
    rslt = new Image();
    rslt.src = arg;
    return rslt;
}

function changeImages() {
    if (!preloadFlag) return;
    for (var i=0; i<changeImages.arguments.length; i+=2) {
        document.images[changeImages.arguments[i]].src
            = changeImages.arguments[i+1];
    }
}