El problema no es tu fuente .. sino de como las librerías GD tratan caracteres de ese tipo (acentos, etc ..)
Parece que tu texto vas a tener que convertirlo a UTF8 antes de ingresar el dato a tus funciones de GD con:
mb_convert_encoding()
http://de2.php.net/manual/de/functio...t-encoding.php
ó
utf8_encode()
www.php.net/utf8_encode
Lo digo por el comentario de php.net al respecto:
Cita: The text string.
May include decimal numeric character references (of the form: €) to access characters in a font beyond position 127. The hexadecimal format (like ©) is supported as of PHP 5.2.0. Strings in UTF-8 encoding can be passed directly.
Named entities, such as ©, are not supported. Consider using html_entity_decode() to decode these named entities into UTF-8 strings (html_entity_decode() supports this as of PHP 5.0.0).
If a character is used in the string which is not supported by the font, a hollow rectangle will replace the character.
http://www.php.net/manual/en/function.imagettftext.php
(Pero realmente por mi parte no lo he probado, mi respuesta sólo son sugerencias).
Un saludo,