Hello,
I can’t check your site because of the protection.
You can do this using below custom code (Theme Options > Custom CSS)
.site-header {
background: url(add_link_to_your_image);
background-repeat: no-repeat;
background-position: center;
}
And don’t forget to set the bg color transparent for all header areas http://prntscr.com/t9fnoz
Regards