Grazie ZACCA94, come sempre gentilissimo... ora che ho corretto, l'errore è cambiato in:

Warning: getimagesize($file) [function.getimagesize]: failed to open stream: No such file or directory in engine.php on line 42

Warning: imagecreatefromjpeg($file) [function.imagecreatefromjpeg]: failed to open stream: No such file or directory in engine.php on line 46

Warning: imagecopyresized(): supplied argument is not a valid Image resource in engine.php on line 47

Warning: imagejpeg() [function.imagejpeg]: Unable to open '/images/thumb_$file' for writing: No such file or directory in engine.php on line 50