I'm sorry that my english is poor and I really have a big problem to search the site here for a solution. I read a lot but I never found an explanation how I can deny access a selection of some pictures in published albums. My Gallery should generally be public, anyway some pictures should only be accessible to registered users. Is it possible to have protected images in a published public album? Or can I password protect single pictures so that only registered users can see them? I have no idea how to do... Thank you for any support!

Please take a look here: https://www.zenphoto.org/news/an-overview-of-zenphoto-users/

Bottom line you can password protect albums but not individual images.

Thank you for your response... I will try to find a solution and hope I will find out how to password protect albums. I found it before, but not again...

You can set passwords for albums, well, on each album in the backend.

Hello,
Here is my trick :
you use 3 albums :

one protected album with pictures you want to hide, using a defined tag on each image

one public album with pictures you want to show, using same tag on each image

one dynamic album using your tag.
This dynamic album is the one you will show !
Are you connected ? all pictures will appear in it
Are you a simple visitor ? only those from public album will !

If you do not want 2 first albums to show, you will need to modify index.php in your themes/yourtheme directory.

I do use this trick, it does work !