Can someone explain how Ubuntu manages wallpapers?

I am confused about how Ubuntu stores user-selected wallpapers.

I have some pictures I use as wallpapers. We'll call them walla and wallb. Both originate from an encrypted drive that is not automatically mounted. walla is the active wallpaper, but is not in ~/Wallpapers, but wallb is.

So I think "Ok, so Ubuntu must have a place where it copied walla", so to experiment I set wallb.png as the wallpaper and then removed it from ~/Wallpapers. This caused the background to disappear. This left me confused as to where it stored walla, because that encrypted drive I mentioned was not mounted.

Could someone please tell me where Ubuntu stores the active wallpaper and how it goes about deciding when to copy user-provided wallpapers to various destinations?