How to set GNOME(a.k..a GDM) greeter (a.k.a lockscreen/login) background(a.k.a wallpaper)?

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Thanks, but I’m on day 4 of NixOS and what you just said is way over my head. I tried following [1] and posted my results. Maybe you could response to that?

[1] GDM Background Image and Theme - #2 by DiCaius