Add 4k png and xcf of simple blue

[khinch]

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/is-there-a-4k-version-of-the-simple-blue-wallpaper/33970/9

[TLATER]

Just to be clear, original work was by @ericsagnes, who published it under CC0 as I understand it, I just imitated it to be as close to the original as I could manage.

@ericsagnes if you stumble across this and still have the gradient settings from the original, a closer match would be cool ;)