Modifica la riga in cui componi l'html così:

Codice PHP:
...
echo 
"[url='".$image_dir.$image_file."'][img]".$image_dir.htmlspecialchars($filenameENT_QUOTES).".".$filetype."[/img][/url]"."\n";
... 
Per approfondire consulta htmlspecialchars()