The way things are now, it looks in your PATH. So basically, the PHP code runs "which convert" to look for imagemagick. Can you arrange for your webserver user to have /opt/local/bin in your path for now? Eventually we'll come up with a UI that lets you configure this, but sadly we're not there yet. As a quick workaround, you can edit kohana/config/image.php and change
$config['params'] = array();
to
$config['params'] = array("directory" => "/opt/local/bin");
that should take care of it for now.