3.7.11 works fine on a different site, but this is on a new website so I may be missing some setting somewhere.
Yes, I have added this to the HTML/CSS Inserts section:
div.header-image-container img {width: 100%;}
and/or
div#imagecontainer {background-size: 100% !important;}
And yes, I have tried all the browser settings in "Style & Configure Layout"
Anyone with ideas?