var theImages = new Array()

theImages[0] = '<img src="images/main_photo01.jpg" width="620" height="100" border="0">'
theImages[1] = '<img src="images/main_photo02.jpg" width="620" height="100" border="0">'
theImages[2] = '<img src="images/main_photo03.jpg" width="620" height="100" border="0">'
theImages[3] = '<img src="images/main_photo04.jpg" width="620" height="100" border="0">'

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(''+theImages[whichImage]+'');
}