You're on the right track!
I checked your site and I see you've setup a background this image with this CSS:
But the image you're using is too small to fit:
What you can do is create a larger image that fits the space you've created for your header image area. Currently, it looks to me like a header image sized 850 x 163 pixels would fit well. You should create an image with the dimensions you want, upload it to your media library, and then replace the image URL in the CSS shown above to the new image URL.