Shoot all Secret Skullshots in all stages of a level to unlock the concept art for that level.

Bunker concept art: Shoot all Secret Skullshots in all the Bunker stages.
Manor House concept art: Shoot all Secret Skullshots in all the Manor House stages.
Ocean Liner concept art: Shoot all Secret Skullshots in all the Ocean Liner stages.
Prison Cells concept art: Shoot all Secret Skullshots in all the Prison Cells stages.
Steel Mill concept art: Shoot all Secret Skullshots in all the Steel Mill stages.
Venice concept art: Shoot all Secret Skullshots in all the Venice stages.
Warehouse concept art: Shoot all Secret Skullshots in all the Warehouse stages.
Warzone concept art: Shoot all Secret Skullshots in all the Warzone stages.