How to: header background image

I’m putting a site together and I’m using the blackbird theme. I need an image to be the background of the header div so that it (the image) will fill in the area right down to the menu-wrapper.

I’ve tried a number of things to no avail, including this which seemed to work for another poster with a similar challenge… but it did not work for me:

#header { background-image: url(https://mrmarketingservices.com/norseman/wp-content/uploads/2012/12/logo_map.gif); }

Any ideas or advice? The theme drops the logo inside the header div inside of another div but this is terrible because it leaves a space below the logo and doesn’t quite stretch to the edges of the header area properly.

You can view the site here to get an idea what I’m talking about:

https://mrmarketingservices.com/norseman/