First of all, congratulations on your new version!
When including ThumbLib.inc.php from my index.php file, the paths to the PhpThumb.inc.php, ThumbBase.inc.php and GdThumb.inc.php need to be relative to the path of my index.php file.
A workaround is to adjust the paths to the included files inside ThumbLib.inc, but perhaps it's better to implement a universal method to determine the right path?