Simple CSS issue

I'm trying to add an image to the background of the jumbotron using CSS, I can successfully type background: URL(image.jpg)no-repeat but nothing happens. The images are loaded up into bootstrap, and I can drag and drop them anywhere I want, but I can't insert them via CSS. What gives?

When I type in a gradient or color, it shows just fine.

No issues here, maybe you can check the online component "Hero Photography" to see how it's done