I have placed a large background image in my static folder, which contains an img folder; I link to it here in the config.toml so I can change my site’s background to this image.
There’s far too less information in your post. But I guess that you should get read of the /static/ part in your URL. Also, the developer console of your favorite browser will tell you which images your document tries to load.
Well yes, you mentioned that. But last time I looked, backgroundImage was not part of HUGO’s config parameters. So it is probably something your theme uses.
But you didn’t even mention that you’re using a theme, far less which one.
Or maybe you use it in a template, but you didn’t say so either. How can anybody but you know what you’re doing to display the background image? As @frjo said:
Otherwise, only a clairvoyant can help you, and those are quite rare nowadays.