Thanks a lot, works perfectly!
The general instructions are just a bit confusing in one aspect. It says "Rename your copied config file(s) to something unique for each file, e.g. add a prefix like "my_" or anything else you wish to use." -- But the new config doesn't show up in the drop-down menu if you add a prefix to the file name, i.e.: prefix_zenphoto-slim.js.php.
It does work though, if you replace "slim" by something individual, like: zenphoto-myconfig.js.php.
At least that's how it works for me ...