function setRandImage(imgId, images) {
  document.getElementById(imgId).src = "/images/" + images[Math.floor(Math.random() * images.length)];
}

var old_onload = window.onload;
window.onload = function() {
  var stock_images = ["stock/arches.jpg", "stock/colosseum.jpg", "stock/pillars1.jpg", "stock/pillars2.jpg"];
  var people_images = ["people/craig_berne-160.jpg", "people/david_brownhill-160.jpg", "people/milt_christensen-160.jpg", "people/roger_harris-160.jpg", "people/steve_berne-160.jpg", "people/thomas_cutler-160.jpg"];
  setRandImage("rand_stock", stock_images);
  setRandImage("rand_person", people_images);
  old_onload();
}