var theImages = new Array() 

theImages[0] = 'image01'
theImages[1] = 'image02'
theImages[2] = 'image03'
theImages[3] = 'image04'
theImages[4] = 'image05'

var p = theImages.length;
if (document.documentElement)
{
  document.documentElement.className = theImages[Math.round(Math.random()*(p-1))];
}