Add image to header background

Hello. I’d like to add an image to the background of my header, rather than have it show white (Highend, Header Layout #3).

Attached is the image I want it to look like – a full size responsive image behind the logo.

Website is http://www.joyfulfamilylife.com
I’m still in maintenance mode though. What information do you need from me to create some CSS?

Thanks for your help! I appreciate it.

-Nicci