[Theme: Simple Catch] Header Image

Can you please help me in how to use image as a header? I’d like to use the same image for my background as a header. Right now, my header is a plain light grey and it looks weird.

I’m a total beginner and this is my first website using WordPress. I’m still getting my website sorted out so excuse the mess. My website is http://www.bestunningmakeupandhair.com

What code should I insert to replace the grey header background with the same background image I already used? Thank you so much and I really appreciate the help.