There is provided an image archiving method for use with a writing target, comprising the steps of receiving a series of captured images of the writing target, detecting difference between first and second candidate received images separated by a predetermined period of time, where additive differences are indicative of writing and subtractive differences are indicative of erasure; upon detecting subtractive difference, temporarily retaining a last candidate image captured prior to the detection, and detecting whether the subtractive difference relative to the retained image exceeds a subtraction threshold amount; and if so, then storing the retained image.