So, those that have tried it so far have all, by default, only uploaded a light background image (unless I prompted them).
Which suggests that it's annoying uploading separate backgrounds by default. So I've changed the logic a bit.
If you only upload 1 background it will be used for light AND dark mode. If you upload both, it will use the individual images appropriately.
If you upload an image with a transparent background by the way, then it'll follow light and dark mode automatically :-) But of course, photos don't tend to have transparent backgrounds 😀
So, if you have a photo that works for both light and dark, you now only need to upload it once. If it's a very bright background I would still recommend finding an alternative photo for dark mode.
When I setup the ability to adjust individual colours I will be making it mandatory to set both dark and light mode colours though if you set, for example, the 'card' background colour then you must do both light and dark... But that's for another day ;)