One of my favourite ones cause NEXON's marks are still fully intact. God, imagine if this game got sucked into by NEXON as well like what happened with MapleStory. Can hate NEXON all you want, but MapleStory is still alive to this day, GunBound is not. That said though, I have zero interest in current MapleStory, so maybe it would have had the same ending.
Also not really a poster or artwork but I also like this GG splash screen from TH era.
Sadly, later they got rid of it and used this instead
... And this one for GBNA
It's actually quite unique that GBNA had their own. The other regions from what I can tell all used the GIS one. Maybe GunBound Planet Battle had their own as well, but that client is completely lost...