Most background images are presented within "Container" widgets on the site's pages.
If you wish to change background on your site's page - please follow these steps:
1. Choose any widget over background image (it can be text, space or menu widget for example). On the left side you may see widget's title within the blue shape (button). In our example it's Text widget. Click on the shape so it will open nesting tree of objects. You need to choose "Container" widget.
2. Once it's done, enter "Design" tab on the right side and press "Edit" button (pencil icon) within the "Background Image" field.
3. System will redirect you to the "Media Library". Choose necessary image and press "Apply" button.
4. Now you can preview new "Background Image" in the Control Panel directly. If you wish to check changes on the live site - save the results and press "View Site" button on the top right side of Control Panel.