can someone point me in the right direction on this? i want a theme that operates just like the showcase on zenphoto.org (with navigation thumbnails on the right when viewing the large image). does this already exist somewhere, or something i have to create from scratch?

All known themes are on our themes page, so yes you have to build your own.

But the nav thumbs are done by the paged_thumbs_nav plugin. The Zenpage-default theme uses them but in a row.
If you don't need them paged you can just use the next_image() loop from album.php on image.ph. Please take a look at our theming tutorial, too.