$(document).ready(function() {
  search_off_image = $('.nav .button').attr('src');
  search_on_image = search_off_image.replace('.gif', '-on.gif');
  search_on_image_preload = $('<img>').attr('src', search_on_image);
  $('.nav .button').hover(function() {
    $(this).attr('src', search_on_image);
  }, function() {
    $(this).attr('src', search_off_image);
  });
});