var randompic = new Array() 

randompic[0] = 'images/lane.jpg'
randompic[1] = 'images/wallsky.jpg'
randompic[2] = 'images/circle.jpg'
randompic[3] = 'images/window1.jpg'

var j = 0
var p = randompic.length;

var preBuffer = new Array()
for (i = 0; i < p; i++){
   preBuffer[i] = new Image()
   preBuffer[i].src = randompic[i]
}

var whichImage = Math.round(Math.random()*(p-1));
function newimage(){
document.write('<img src="'+randompic[whichImage]+'">');
}