function switchImg(el, brighten) {
    var img = el.childNodes[0];
    var prefix = "/i/" + el.id;
    var suffix = (brighten) ? "-rollover.jpg" : ".jpg";
    var filename = prefix + suffix;
    img.src = filename;
}

function preload() {
    i1 = new Image();
    i1.src = "/i/head-military-rollover.jpg";
    i2 = new Image();
    i2.src = "/i/head-fire-rollover.jpg";
    i3 = new Image();
    i3.src = "/i/head-cycle-rollover.jpg";
}