How do I change the background image?

I have uploaded the theme and can see the in the customise section the ability to change the Body Background Image but the site does not update. It always shows the default image that was provided with the theme. I have tried in an incognito browser but it still does not update.

I have tried to add images to sliders and they do not work either.

The page I need help with: [log in to see the link]