Cavalary: There's something to be said about the simplicity of this one from Heretic Kingdoms: The Inquisition.
And then there's the classic fantasy art from Baldur's Gate 2.
Oh crap, somehow I forgot to download BG2 wallpapers when I backed up the game. Some of these look incredible!
Warloch_Ahead: Deus Ex and Thief games have pretty good wallpapers included if you want a dark theme.
Far Cry 2 has some pretty natural landscapes, but a lot of them are just "there's this guy, you see... and he has a gun! And there's FIRE!", and it's orange and brown.
Jade Empire, Pillars of Eternity, and Arcanum have beautiful artworks.
Some of the worst wallpapers though are ones that have the low poly 3D character models posing or are otherwise just screenshots of the game. Like Crysis man looks good at first glance because it's well detailed enough that closer inspection reveals the polygons and smudgy textures, but something like Painkiller or Unreal Tournament 2004 look real dated. Or have a lot of white space like Dragon Age Origins and WH40k Space Marine.
Yeah, 3D characters from old games get the short end of the stick when it comes to wallpapers, if they are portrayed in all of their polygonal glory.