if (document.images) {
img1 = new Image();
img1.src = "images/link1a.jpg";
img2 = new Image();
img2.src = "images/link2a.jpg";
img3 = new Image();
img3.src = "images/link3a.jpg";
img4 = new Image();
img4.src = "images/link4a.jpg";
}

function linkOn(num)
{
document.images['link' + (num)].src = "images/link"+(num)+"a.jpg"
}

function linkOff(num)
{
document.images['link' + (num)].src = "images/link"+(num)+".jpg"
}

function switchBack () {
var picnum=(Math.floor(Math.random()*4)+1);
document.getElementById("homecontent").style.backgroundImage = "url(images/frontback"+(picnum)+".jpg)";
}