Shoot all Secret Skullshots in all stages of a level to unlock Concept Art for the area.

Art	How to unlock
Bunker Concept Art	Shoot all Secret Skullshots in all 6 Bunker stages.
Manor House Concept Art	Shoot all Secret Skullshots in all 6 Manor House stages.
Ocean Liner Concept Art	Shoot all Secret Skullshots in all 6 Ocean Liner stages.
Prison Cells Concept Art	Shoot all Secret Skullshots in all 6 Prison Cells stages.
Steel Mill Concept Art	Shoot all Secret Skullshots in all 6 Steel Mill stages.
Venice Concept Art	Shoot all Secret Skullshots in all 7 Venice stages.
Warehouse Concept Art	Shoot all Secret Skullshots in all 6 Warehouse stages.
Warzone Concept Art	Shoot all Secret Skullshots in all 6 Warzone stages.