couldn't find but a few picks with google searches.. was hoping for a better resource.
The Library of Congress is a good start. I've searched for 'slavery' and selected photos/drawings/prints as category.
I don't recall ever seeing pictures of slaves at work, or slaves being (mis)treated as slaves, but maybe this archive has some.
Keep in mind that photography was relatively young before slavery was abolished and that the technology at that time required an exposure time of multiple minutes, thus making actual photos of working slaves unpractical if not impossible.