A mental picture one stores in their memory; later used to fap with. (Take a snap pic for the snap file) A snap file can be accessed whenever; when you're in the pooper, bed or even at the tire shop.
Winthorp- Oi mate! you see that milf over there? Redginol- Oh, datass by the slurpee machine? Winthorp- yeah, we'll put that one in the snap file..