var Images = new Array()

Images[0] = 'images/leftimg1.jpg'
Images[1] = 'images/leftimg2.jpg'
Images[2] = 'images/leftimg3.jpg'
Images[3] = 'images/leftimg4.jpg'
Images[4] = 'images/leftimg5.jpg'

var b = 0
var c = Images.length;
var preBuffer = new Array()
for (a = 0; a < c; a++){
   preBuffer[a] = new Image()
   preBuffer[a].src = Images[a]
}
var whichImage = Math.round(Math.random()*(c-1));

function showImage(){
document.write('<img border=0 src="'+Images[whichImage]+'">');
}