var theImages = new Array() // do not change this

theImages[0] = '<img src="images/mast_p_random_1.jpg" width="530" height="250">'
theImages[1] = '<img src="images/mast_p_random_2.jpg" width="530" height="250">'
theImages[2] = '<img src="images/mast_p_random_3.jpg" width="530" height="250">'
theImages[3] = '<img src="images/mast_p_random_4.jpg" width="530" height="250">'
theImages[4] = '<img src="images/mast_p_random_5.jpg" width="530" height="250">'

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]+'');
}