Koken, whilst not mature yet, is developing to be a very good choice for a CMS for artists and photographers, not least because of the excellent responsive performance for displaying pictures.
However, at the moment, it does not natively provide a contact form.
We can recommend starting from that which Jürgen Mackiol has kindly made available via his site.
You will probably want to make some tweeks to improve the appearance of the form (Jürgen has given the idea, not the actual appearance of the one he created). Also, note that eregi, which is used in webmailer.php, is deprecated so the eregi needs to be replaced with preg_match("/^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\\.[a-zA-Z0-9-.]+$/"