window.addEvent('domready', function()
{

var tb_images = $$('.tbimg');
tb_images.each(function(img) {
  var src = img.getProperty('src');
  var ext = src.substring(src.lastIndexOf('.'), src.length);
  var hl = src.replace(ext, '_hl' + ext);
  //preload the image
  document.createElement('img').setAttribute('src', hl);
  
  img.addEvent('mouseenter', function(){ 
    img.setProperty('src', hl); 
  });
  img.addEvent('mouseleave', function(){ 
    img.setProperty('src', src); 
  }); 
});

});