Hey buddy,
What I did was go to Custom CSS in the template and I added the following code
#header-section:before {
background-image:url('http://www.yourwebsite.com/images/banner.jpg') !important;
background-size:900px 120px;
background-repeat:no-repeat;
background-position:center;
}
Add whatever height and width you want. Note that this creates a background image in the header position, so it might not be exactly what you want, but you should be able to tweak the CSS to your needs. This is actually the desired effect I wanted as I wanted my header image to be behind the menu as well.
Hope this helps.