var theImages = new Array()

theImages[0] = 'images/b1.jpg'
theImages[1] = 'images/b2.jpg'
theImages[2] = 'images/b3.jpg'
theImages[3] = 'images/b4.jpg'
theImages[4] = 'images/b5.jpg'
theImages[5] = 'images/b6.jpg'

var j = 0
var p = theImages.length;
var preBuffer = new Array()
for (i = 0; i < p; i++){
   preBuffer[i] = new Image()
   preBuffer[i].src = theImages[i]
}
var whichImage = Math.round(Math.random()*(p-1));
function showImage(){
document.write('<img src="'+theImages[whichImage]+'" class="img_border">');
}