adding header image in Barthelme

I’m wanting to use Barthelme as a theme, but I can’t figure out how to get my header image to work. I managed to remove the header text but I can’t get the image to be seen. I tried to add it to the stylesheet, but all I get is the Barthelme color gradient. The stylesheet is laid out very differently than the last few themes I’ve used so I’m a bit lost.