I’ve checked your site and it seems you’ve inserted the image as a “single image”. Instead of this create a new row and from the edit area go to the “Background Style” tab and “Background Style”. There you’ll have multiple options of setting up your bg.
I have set it up for you as an example and now you can see the content is a 700px spacer. You can proceed and switch the spacer with any content you want over your background.