I ran into this issue recently, and there could be many different fixes for this problem, from CSS, incorrect HTML, etc. This fix applies more if using certain dark mode extensions. If you run into this issue where a background image is not showing on a website, first try adding an exception to your dark mode extension. For instance I run Dark Reader, and this was causing background images not to display properly on Firefox.
Here is a quick video of what I mean:
Simply toggle the extension off for the website and you should see your background image now.