How to remove all permissions to "Everybody" group

Hi,

I want that only registered users can see my images. And we you arrive on the landing page of my gallery, you have to give a user/password. That's ok ! But I just discovered that everybody could see my images, from a direct Google link... However, "Everybody" has no rights in the Permissions section. But in reality, "Everybody" can see all versions of my images... Do you know how to remove a right that does not exist yet ? I thought that "Everybody" has no rights at all, and then you can add some. Thank you for your help !

Gallery version (not just "2"):2.3.2
PHP version (e.g. 5.1.6):
PHPInfo Link (see FAQ):
Webserver (e.g. Apache 1.3.33):
Database (e.g. MySql 5.0.32):
Activated toolkits (e.g. NetPbm, GD):
Operating system (e.g. Linux):
Browser (e.g. Firefox 2.0):