I got this to work in mine by inserting the following code. I copied it into the section they have already made for getting an image in the body. Its around line 1607 or so in the theme.scss.liquid.
Here's the snippet I used:
/* remove background colour from header */
/* place background image into header */
background-size: 65% 40%;
Here's my website if you want to see how it looks:
There's still a little issue with sizing on the mobile app, but I can live with that until I figure out how to fix that.
Good luck with yours!