Adding Custom Image into Full Header of Theme

Hi there, I would like to add an image which goes across the whole header of my subsite.
Currently it has white borders around it.

Also I'm not really clear how subsites work with themes, i can only see a child theme for my main site in the wpcontent folder, if I change the CSS there won't it change it for the subsite?

I'd like to create a different header for the subsite.

You can get to the Danielsite through the WPMUDEV access into the Network Site.

Any help appreciated.

Daniel