Featured image not showing

Hi,

On my website’s “inner” pages, I can’t for the life of me get the featured images to show.

One example is https://festindesperance.org/don/

On https://extendthemes.com/knowledge-base/one-page-express-theme/knowledgebase/background-images-different-for-each-page-of-the-site/, I read that “for the background image to display, you have to previously have selected the background type to Transparent, in the content section (Page Content -> Manage Page Sections).”
Well, I can’t find such settings, and I played with all kind of setting combinations in the Customizer, with no success.

Would anyone provide me with pointers ?