- Create a new Custom page and leave the content area blank
- Set this page as your homepage. You might also consider setting this page to hidden (by clicking the green "visible" indicator on the page) so that it doesn't appear in your navigation. It can still be visited again by clicking your site title or logo.
With a blank Custom page, your site's background image will be shown with full opacity with only your header showing so your visitors can navigate to your other pages.