Event.observe(window, 'load', function() {
    var images = $$('img.thumb');
    images.each( function(img)
      {
      Event.observe(img, 'mouseover', function()
        {
        var newImage = img.src.replace(/thumb/,'full');
         $$('img.mainImage')[0].src = newImage;
        });
      });
    });