I just think it could be one particular way to integrate 2 or more different sites. Principle: Joomla site has own image directory and Wordpress another, but all images are shown on the zp site.
Also, as I understand, some files could not be displayed when using external album directory. In this case these files would be in the zp's "own" directory.
We officially support only one "albums" folder but if you figure something out feel free to post your solution here. The only way to use external source is to use the "youtube" trick posted on our troubleshooting. But that will not let you benefit from the image resizing.
acrylian: "Maybe it could be done via symlinks."
YES, it works! No need for additional ZP installation. Just create the needed symlinks into albums folder. You need a php file (e.g symlink.php) with content
Upload this php file into your albums folder (or any subfolder), chmod 0777 both file and albums (or albums/whatever) folder (maybe parent folder, too) and then point your browser to this file (if this is in albums folder: http://www.yoursite.com/your_zp_root/albums/symlink.php )
Blank screen means that symlink is created successfully. Even if you cannot see the symlink in ftp or cpanel, you can see new album in zenphoto and you can upload there!
Then chmod changed folders back.
Now I can see both the "original" zp site pictures and the Joomla site pictures with one single zp installation. And other media files as mp3 are accessible, too.