The magical bank in a persons head where one can save,deposit or withdraw memorable mental images*,usually with the intention of having an epic wanksterbate over them later.
*i.e: tittys and ass
A: "Yo,check out the mega miniskirt on the coca-cola rep,shes goin in the spankbank"
B: "yea i'll withdraw one later."
A: "Bro that girl over there got some big big tittys"
B: "Yea dude i made a deposit into the spank bank like 8 miniutes ago."