Allow for image helper to use background image instead of image tag

Yea I actually wasn't sure what the best kind of assertion to do there. I actually don't care as much about the type as I do it's existence. Because it must have both attributes to work. I tried digging around for other assertion types but I couldn't really find what I was looking for.

Also for asserting as a number, that is sort of inconsistent for how we do attributes everywhere else, oder?