Hey, it's very much possible, as mosefthebrosef said, and you do not even need to recompile the Game.pak (I think doing so will cause issue with later patches etc, but not sure.)
Anyway. You do need to unpack the Game.pak, and you do need to decompile the loadingScreen.swf file.
I used the JPEXS free flash decompiler. It is open source software, so no watermarks nonsense. All it takes just couple of clicks to select and replace the loading screen image within and then save the loadingScreen.swf again.
Then dumped the said loadingScreen.swf file into ...\steamapps\common\Divinity Original Sin 2\Data\Public\Game\GUI folder, that I had to create inside the game's directory. Worked like a charm. The path for GOG version is likely different, though.
(My Lohse avatar wasn't a redhead, so it was bugging me, please don't judge
I think the concept art was amazing otherwise!)