function imageOn(imageName) {
    if (document.images) {
        currentImage = document.images[imageName];
        currentImage.src = imageName + "_on.jpg";
    }
}

function imageOff(imageName) {
    if (document.images) {
        currentImage = document.images[imageName]
        currentImage.src = imageName + ".jpg";
    }
}

//