Including pictures in our Newsletter
Hi Felix and welcome to CiviCRM Stack Exchange. 

Your problem could have a number of reasons, e.g. the following:


your server does not allow access via http for anonymous users (could
be protected by htaccess, a vpn...)
you do not have a ssl certificate or it is self-signed


If that is not the reason please try to provide some more information such as:


are images not displayed for all recipients or only certain ones?
does it happen with all mail clients/services or only certain ones
...