Thanks for your answers. I know I have to do this, but I don't know how.
These are my current rights. They are correct for what I want.
How do I change the rights for wiki:
I do only want to give rights to the logo, but nothing should be visible if not logged in.
I don't see any way to enter the namespace: wiki:
If you mean I should give rights to [root] - I believe this would make my wiki readable for not logged-in people.
When I give more rights to "start" it does still not show the logo.