What must I add to my wp-config to use ZenPhoto functions in WordPress? I need this only for my conditional title tag in my header.

Don't know anymore. Changes to WP have made this kind of thing dificult. So much so that we have abandoned WP for the Zenphoto site and are usng Zenphoto itself,

Is there anyone that could provide me a solution?

As my collegue said, No. Wordpress calls its theme pages within function which virtually destroys the Zenphoto global environment required if trying to use the "Zenphoto as a plugin" way.

There is no easy way to workaround this as both WP and ZP are independent CMS that naturally are not meant to be "joined". There are WP plugins like Zenphotopress that offer the possibility of including images from Zenphoto.

If you want to use both you need to use them independently and make your site's look to hide that fact optically.

As my collegue said, you can also use Zenphoto to do it all as you see on our own site.