var theBanners = new Array()

theBanners[0] = '../banners/bobstg-ban1.jpg'
theBanners[1] = '../banners/bobstg-ban2.jpg'
theBanners[2] = '../banners/bobstg-ban3.gif'
theBanners[3] = '../banners/bobstg-ban4.jpg'
theBanners[4] = '../banners/bobstg-ban5.jpg'
theBanners[5] = '../banners/bobstg-ban6.jpg'
theBanners[6] = '../banners/bobstg-ban7.jpg'

var j = 0
var p = theBanners.length;

var preBuffer = new Array()
for (i = 0; i < p; i++){
   preBuffer[i] = new Image()
   preBuffer[i].src = theBanners[i]
}

var whichBanner = Math.round(Math.random()*(p-1));
function showImage(){
document.write('<img src="'+theBanners[whichBanner]+'">');
}