var theImages = new Array();

theImages[0] = 'red.jpg';
theImages[1] = 'blue.jpg';
theImages[2] = 'yellow.jpg';

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));