How do I make the header background picture below header menu look bigger?

Hi my question is how can I make the header background picture that is between the header menu and the body of the website look bigger? I mean, even clicking on Demo Import button I am not getting the same 100% look as the demo… On the preview of this theme we have this amazing background image at the header of the page but I am not finding any way to replicate this in my actual website… How can I make this space bigger?