Need help adding an extra image/header below my navigation bars

Trying to make a nail salon site for my friend – https://www.a1mix.com – ignore the doamain, it is a temp one I am using.

I am trying to add an extra header-type image below my navigation bars (home, about, location, etc) and I couldn’t find a way to do it.

Also, where can I edit the width of the navigation bars? I am trying to make it so it will fit the full width of the site. Thanks for the help sorry I am new to wordpress