Make Header Full-Width Across Website

Website: http://www.bookaddictsguide.com
Wordpress version: 4.1.1
Child Themes in use

Hello! I rearranged my website layout and would like to stretch my header image across the full-width of my website. I’d done it once before a few years ago but once I changed it, I can’t seem to put that into effect again. I’d like it to span all the way across the top of the page so the whole header image creates a nice top-border effect.

Thank you in advance for any help!