var preloaded_imgs = new Object();
var saved_img = new Image();

function preload_img () {

  if (document.images && preload_img.arguments.length) {
    for (i = 0; i < preload_img.arguments.length; i++) {
      preloaded_imgs[i] = new Image();
      preloaded_imgs[i].src = preload_img.arguments[i];
    }
  }
}


function rollover_img (img, img_no, action) {

  if (action == 'swap') {
    saved_img.src = img.src;
    img.src = preloaded_imgs[img_no].src;
  }

  if (action == 'restore')
    img.src = saved_img.src;
}