Symbolism, Imagery, Allegory
The portrait of Will's grandmother isn't the only symbol of history coming back to haunt the present. Bulstrode's dodgy history comes back to haunt him in the form of Raffles. Raffles is the only other person who knows about the source of Bulstrode's money, and Bulstrode just can't seem to get rid of him. He only feels guilty for his past actions when Raffles comes around, so it could be that Raffles is meant to be seen as a symbol of Bulstrode's conscience.