Background "post" image instead of color possible?

[sysadmininator]

I have tried to modify my post header to make the background of an image instead of a color but I can't seem to get it to work. I don't see anything that says it should be possible so I'm not expecting to pull a feature out of midair ;) but I do want to ask to make sure I'm not missing something.

I have tried adding the following:

image: img/apple-touch-icon.png

and

image: 
   background: img/apple-touch-icon.png

Admittedly, this idea/setting is borrowed from another Jekyll project (https://github.com/mmistakes/jekyll-theme-hpstr) and I am not going to pretend that I know enough about how these projects are built to assume that every Jekyll theme has the same features. If there is a place I should reference with each theme that spells out what functionality is available (I browsed the source for a couple but nothing jumps out to my untrained eye), I would love to "learn how to fish" on this one as well. Thanks!