I really loved the artwork as well, so I reached out to Violaine Briat to see if I could get copies in the highest possible resolution and quality, and she was kind enough to provide. Since you want them for the same reasons I did, I think she'd be OK with me passing the files onto you, so here they are. I'll happily take down this link if Violaine or Nifflas ask me to, though.
[mod]I have removed the link because it is public for the whole world now. For such sharing you really first need permissions and then share. I hope you understand. I have no hard feelings and I do approve sharing this after you have permission, but this still remains copyrighted material for a game. We can't take any chances here.[/mod]
Edit by Sergio: I seriously considered this LPChip, but I get a strong impression that both Violaine and Nifflas are quite busy people. I feel that asking could be a burden on them and complicated as I don't know who actually holds the copyrights to these images. So I'll just keep them to myself and inform others that clicking the 'HD' links on this page (http://www.steamcardexchange.net/index.php?gamepage-appid-248190) leads to images openly hosted on the Steam website that are fit for desktop backgrounds, despite containing significantly more JPEG artefacts.