function update_background(img) {
    document.getElementById('Main').style.backgroundImage = 'url(' + img + ')';
}

function toggle_buttons(number) {
    document.getElementById(number).src = '../images/no.' + number + '.on.gif';
    for (i = 1; i <= 5; i++) {
        if (i != number) {
            document.getElementById(i).src = '../images/no.' + i + '.off.gif';
        }
    }
}