$(function() {
  $("img.rollover")
    .each(function() {
      var baseImage = $(this).attr("src");
      var ext = baseImage.substring(baseImage.lastIndexOf('.'), baseImage.length);
      var overImage = baseImage.replace(ext, '_over' + ext);
      new Image().src = overImage;
      $(this).attr({basesrc: baseImage, oversrc: overImage});
    })
    .hover(function() {
        $(this).attr({src: $(this).attr("oversrc")});
      },
      function() {
        $(this).attr({src: $(this).attr("basesrc")});
    }
  );
});