Art-Nude series from Cydia is a winterboard addon/lockscreen theme which gives you a single tap unlock option with an artistic (but nude) pic as the lockscreen background. Screenshot above shows one of the ArtNude series wallpapers.
Search for ‘Art nude’ or ‘ArtNude’ on Cydia. Available via Winterboarder repository.
(Legal-note from the source: You must be above18 for this series!)