Here is the Requirements for the Gunslinger mode in Extras,
complete these tasks to unlock art for that specified level.

Wrong Train AnimaticComplete The Wrong Train with 65% or
more Accuracy
Enemies 1Complete Boneyard with 65% or more Accuracy
Enemies 2Complee Showdown with 20 or more Melee Kills
VehiclesComplete Deadfall with 20 or more Headshots
Allies art, Bosses ArtComplete Curse Of The West on
Shootist Difficulty
WeaponsComplete Morning After with 70% or more Accuracy
Environments 1Complete Darkwatch Outpost with No Deaths
Environments 2Complete Invasion on Deadeye Difficulty
Environments 3Complete Curse Of The West in 10 Minutes
or less
Attract Mode MovieComplete Dead Light Prism with 80 or
more Kills
Highmoon TrailerComplete Rescue in 15 Minutes or less
Rescue Intro MovieComplete Hangtown with 20 or more
Headshots
Cowboy AnimaticComplete Torture Maze with 30 or more
Melee Kills
Showdown AnimaticComplete The Right Train on Shootist
Difficulty
Evolution of JerichoComplete the Single Player Campaign
on any difficulty
Darkwatch ArchivesComplete all Gunslinger levels on
Deadeye