I found this problem also, I am not an expert as others are on this forum, but I can tell you what I did to solve this problem.
I think firstly from the forum that what we are working with is a new upgraded template as the questions on here earlier do not relate to this problem.
When you look at this with firebug it is seen that the background in the header is called from gkheader and not gkheaqder.bg as it seems to have been in the old template, and you will note also that the image is under gkHeader.bg as well.
So if you go the the template.css file for this template you will find that the gkHeader is calling the colour #eee which is the grey background you have on your site. To solve this I replaced the colour being called with the line from gkHeader.nobgthat sends to the image, and just removed the .nobg and it worked.
I gather this may be a change/error that is not documented yet as the template is so new.
I then did the same with the other images for mobile and tablet. This seemed to work OK on PC but not on the other devices which I have placed a question to in this forum but as yet have not had a reply. So I am trying different things to see if I can get it to work properly until I get my answer - which I hope wont be too long.
I trust that this will help you and that I have not led you astray as I am not an expert like the others on this forum.
All the best.
Sandra