The document-based SwiftUI example app (https://developer.apple.com/documentation/swiftui/building-a-document-based-app-with-swiftui) doesn't specify a launch image.
It would seem per the HIG that the "pinkJungle" background in the app would be a decent candidate for a launch image, since it will be in the background when the document browser comes up.
However when specifying it as the UIImageName, it is not aligned the same as the background image. I'm having trouble figuring out how it should be aligned to match the image. The launch image seems to be scaled up a bit over scaledToFill.
I suppose a launch storyboard might make this more explicit, but I still should be able to do it without one.
This is the image when displayed as the launch image:
and this is how it's rendered in the background right before the document browser comes up: