I have a site I wrote some time back now, that has a gallery I embedded. But it's not very user friendly. Zenphoto looks more feature rich, so I was hoping to swap the galleries. So can that be done, or does a whole site need to be build using Zenphoto?

Thanks in advance for any advice.

This article should cover it ;-)
http://www.zenphoto.org/news/integration-with-existing-sites-and-other-cms/

@fretzl - thanks for the link. Sounds like the proper answer is no then Are there any decent quality and easy to use simple galleries anyone knows of?

Thanks again

you don't tell anything about your actual site and what are your needs.
you can create a new site using zenphoto and some plugins as zenpage (blog and pages management) and migrate your actual content, or using zenphoto in your actual site as describe above.