Hi David,
There is no setting in the Customizer to do this, so you need to use some custom CSS to target the header.
This topic covers this.
In regards to it being full width, that simply depends if you have your site set to full width under General >> Layout. http://forum.wpbeaverbuilder.com/support/q/how-to-add-background-image-to-site-header/
HTH, Dave