Hi Happy Girl
Unfortunately the url you've provided is behind a maintenance plugin so we can't see the site.
From what you've described though it sounds as if you need to upload a bg image in the color control tab and then set a color for the page canvas area. Go to the color control tab, upload your image and select with tiled or supersize bg to get it to cover the whole page.
You should now have a full page bg image which goes behind the content. Then scroll to the top of the color control tab and you'll see three color options, select a color for the 'content background' and hit save. Then you should have a page with a background image showing only at the sides and a solid color in the middle.