There is a thin white rectangle about 2 1/2 inches by 8 1/2 inches in the center of my header on each page. I don't know how it got there. It appeared, I thinkn, after I used the logo feature in the upper left corner to successfully place our logo in that corner.
It seems to be part of the styling of the theme. You might be able to get around that the header type for the page to "no header", then uploading your background image to that and adding text/title elements to the section.
