function swap(img, ext) {
        document[img].src = '/images/nav_' + img + ext + '.gif';
}

function preload() {
  // Don't bother if there's no document.images
  if (document.images) {
    document.loadedImages = new Array();
    // Loop through all the arguments.
    var argLength = preload.arguments.length;
    for(arg=0;arg<argLength;arg++) {
      // For each arg, create a new image.
      document.loadedImages[arg] = new Image();
      // Then set the source of that image to the current argument.
      document.loadedImages[arg].src = '/images/nav_' + preload.arguments[arg] + '.gif';
    }
  }
}