Hello Marieke,
Thanks for writing in!
I can see that you are using the Layout - Fullwidth page template while you have created the page with WPBakery Page Builder. Please change the page template to Blank - No Container | Header, Footer instead. For more details about the different page templates in the theme, please check this out:
And then you can add a background image to the whole page in X > Theme Options > Layout and Design > Background Options and insert a background image. By the way, you can change the background color of the navbar in X > Theme Options > Renew.
By the way, I also noticed that you are using X theme 6.3.6 and Cornerstone 3.3.6. Please be advised that we have released X 7.2.3 and Cornerstone 4.2.3 already. It is now available in automatic updates. You can now update to this latest version. This latest release contains fixes for several issues so be sure to check out the changelog (http://theme.co/changelog/).
After doing the updates, always remember to clear all caches (if you are using WP Rocket, WP SuperCache, or W3 Total Cache) when updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
Kindly let us know how it goes.