If I uncheck "visible" for a single image, Zenphoto just adds a CSS class "not_visible" to the image.

Why the image data is outputted if the image isn't visible?

I would like to hide (from the html too) a image, if user doesn't got album editing rights and the image isn't checked to visible.

It's a security issue too.

Because if you are loggedin as an admin with the proper rights items set to not published are visible. This way you can mark them on your theme.

Recommended read:
http://www.zenphoto.org/news/troubleshooting-zenphoto#13
http://www.zenphoto.org/news/troubleshooting-zenphoto#46

Thanks for your reply.

I haven't logged in as a administrator. I have created a user with minimum privileges to browse a gallery (the gallery is hidden for non-users) and logged in with that.

We don't want to hide whole albums: inside of a normal album some images have restrictions so we would like to hide these images from users with a minimum rights.

There is no direct right on image level for various technical reasons. Please read the links I provided plus:
http://www.zenphoto.org/news/an-overview-of-zenphoto-users