var theImages = new Array() 
  theImages[0] = 'pictures/top_0.jpg'
  theImages[1] = 'pictures/top_1.jpg'
  theImages[2] = 'pictures/top_2.jpg'
  theImages[3] = 'pictures/top_3.jpg'
  theImages[4] = 'pictures/top_4.jpg'
  theImages[5] = 'pictures/top_5.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]+'">');
}