var theImages = new Array() 
theImages[0] = 'images/ers_kopf1.jpg'
theImages[1] = 'images/ers_kopf2.jpg'

var j = 0
var k = theImages.length;

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

var whichImage = Math.round(Math.random()*(k-1));
function showImage(){
document.write('<img src="'+theImages[whichImage]+'" alt="" />');
}