Is it planned for the future to add an option for the users to register by themselves with let's say some options like "Approve account by admin" or "Confirm by email" or anything like that.
I've read somewhere that there's a custom user version that allows users to register by themselves, not wait for the administrator to create new account for them but if I decide to use that I'll be having problems with keeping zenphoto up to date and using the latest versions and upgrades.
Thank you

You could make your own custom "spam filter" that filtered out e-mail addresses that you have approved.

This sounds like something PhpGedView does. New visitors can register themselves and be approved by the site Admin. Optionally, the visitor can be pre-approved with preset user/password by Admin. It's a lot more complex setup (given the app's purpose of being for genealogy) so there's a user table and admin features. Quite too much for ZP.

So far in ZP, I've only seen the password appear on my site, never username field. But, not played much with that feature yet. I email visitors the common shared password.