Hello Eddydiver:
Not to sure if this is what you are looking for.
admin/includes/languages/english/header.php.

you should make a copy of that file (header.php) and place it in your custom folder (ie the one you made when you first installed zen)
admin/includes/languages/english/custom/header.php.
then make your changes look for
define('HEADER_LOGO_IMAGE', 'logo.gif');

note your image will also need to
includes/templates/costum/images

nomad
PS custom is what ever name you gave it. Make sure you name it thought the project otherwise it will not work right