I forgot to mention this is in main.css.
Thanks for the help! :)
But it has no effect. I deactivated the folling lines in the LocalSettings:
## The relative URL path to the logo. Make sure you change this from the default, ## or else you'll overwrite your logo when you upgrade! # $wgLogo = "";
[...]
# $wgLogo = "/skins/monobook/logo.jpg";
and did the changes in the main.css
I also deleted the logo.jpg picture