@FokkeZB I'll consider adding this as an option if there is more demand for it, but in the meantime the best way to solve this would be to use a Child Theme, copy the header.php file to your Child Theme, and then modify this line by removing independent_publisher_is_multi_author_mode() &&. That will give you the small top-left logo even when multi-author mode is disabled.

That's also a good alternative for how I did it now (the other way around) but an official option would be great.

However, this would involve more than just a checkbox if you'd still want to have a different image in de sidebar. It would need another image field.