var theImages = new Array()
theImages[0] = 'img/CMbl.png'
theImages[1] = 'img/CMo.png'
theImages[2] = 'img/CMg.png'
theImages[3] = 'img/CMp.png'
theImages[4] = 'img/CMblu.png'

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]+'">');
}