4 million slaves. They had years to take pictures of any whipping scars, and they had Massachusetts abolitionists just scouring the world for proof to support their claims.
I suspect the truth is that this sort of abuse was extremely rare, and that the people trying to claim it was extensive and common, are liars.
Bingo. Scarred up slaves were worth less on the market. How many people do you know willfully damage their own property? Sure there are some people like that, but they are rare. This is a well known case of a particularly sadistic owner. That is of course why the picture was taken and used for propaganda purposes. Now watch the usual liars try to twist the factual statement that this kind of treatment was rare into "you're defending slavery!" These are the same kind of people who claim everything under the sun is "racist" today.