arrBanners = [];
arrBanners.push({
			percent: 100,
			image: "http://ad.au.doubleclick.net/ad/N799.138546.OYSTERMAG/B5488942;sz=300x250",
			link: "http://ad.au.doubleclick.net/jump/N799.138546.OYSTERMAG/B5488942;sz=300x250"
				});
arrBanners.push({
			percent: 0,
			image: "http://oystermag.com/wp-content/themes/oyster-3/advertising/med-rec-02/metals-01.gif",
			link: "http://www.youtube.com/watch?v=kxCT6Iwld80&feature=related"
				});

function getRandomBanner() {
  var curr = 0;
  var rand = Math.round(Math.random()*100);
  for (var i=0; i<arrBanners.length; i++) {
    curr += arrBanners[i].percent;
    if (rand <= curr) {
      return '<a href="' + arrBanners[i].link + '"><img src="' + arrBanners[i].image + '" border="0"/></a>';      
    }      
  }
  return '<a href="' + arrBanners[0].link + '"><img src="' + arrBanners[0].image + '" border="0"/></a>';
}

document.write(getRandomBanner());