Add a background image

I inserted a background image using the customize dashboard, but it's not showing up. The content container is still fullwidth. do I need to change that setting somewhere in order to see the background image? My client really doesn't want the white content to span the whole page and would rather see it in a container with a background image behind it. I assume this is possible since 'background image' is already part of the customize dashboard... what am I missing? Thanks! http://www.certapropaintersminneapolis.com/