var ads = [
	{image:"img/ads/NMFN.jpg",link:"http://www.northwesternmutual.com/"},
  {image:"img/ads/directtire.jpg",link:"http://www.directtire.com/"},
	{image:"img/ads/harpercollins.png",link:"http://www.harpercollins.com/"},
	{image:"img/ads/Bose_logo.jpg",link:"http://www.bose.com/"}
];

var i = Math.floor(ads.length*Math.random());
window.onload = function () {
  document.getElementById('ad').src = ads[i].image;
  document.getElementById('adlink').href = ads[i].link;
}