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

$$('img.mo').each(function(img) { 
  var src = img.getProperty('src'); 
  var extension = src.substring(src.lastIndexOf('.'),src.length) 
  img.addEvent('mouseenter', function() { img.setProperty('src',src.replace(extension,'-mo' + extension)); }); 
  img.addEvent('mouseleave', function() { img.setProperty('src',src); }); 
}); 
});