var theImages = new Array()
theImages[0] = 'images/index1.jpg'
theImages[1] = 'images/index2.jpg'
theImages[2] = 'images/index3.jpg'
theImages[3] = 'images/index4.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]+'">');
}