Flagellation of a Female Samboe Slave, 1796, William Blake
from Narrative of a Five Years Expedition against the Revolted Negroes of Surinam
As these images became more elegant, they also make clear comparisons to Angelica or Andromeda—virtuous targets of heroic rescue. But also, of course, sado-erotic pinup girls.
(Now: why focus on torture as opposed to service or auctions? I think the abolitionist logic was that torture scenes like these were shocking, whereas most of their audience had already seen slaves being working and/or being sold. But those themes come back later on.)