Did you get this working? I couldn’t tell if you did or not after reading your reply.
If not, just add a link that will directly override your default logo. You can add the smaller logo in the responsive section on line 3344 –
.header-image .site-header,
.header-image .site-header .wrap {
background: url(http://mobilewetten.org/wp-content/uploads/logo.png) center top no-repeat !important;
}
Add/Replace whatever link you want in the link area I posted above.