Photographs or any visual representation of slavery can have incredible effects on the viewer that other representations, particularly text, cannot have. One might describe in gruesome detail the horrible practices of abuse and torture that existed on plantations in the United States or in Jamaica or in South America, anywhere that slavery existed, and the horror is palpable and disturbing. But they lack the power and focus of visual images for a variety of reasons.
One, and it was mentioned in the previous answer, is the way that a visual representation allows the viewer to identify as another human being with the slave or slaves in the picture. Sometimes this can inspire more empathy than any description or account, no matter how well written. As visual animals, humans tend to have more of a visceral reaction to visuals than to other forms of presentation.
The second important thing that pictures can contribute are the incredible contrasts between the manner and posture of people enslaved and free people. The juxtaposition of free man and slave that often occurs in visual representations of slavery is often an extremely powerful one because the differences are often understated but also often quite powerful.
I think illustrations on slavery say more than we could ever explain ie. a picture is worth a thousand words.
The pictures of slavery represent in themselves the mistreatment and disregard of slaves as humans. I seriously feel that some slave owners didn't acknowledge these people as that- people. The images suggests the frame of mind that so many people in our country were brainwashed into trying to believe as right. Photos of slaves during and off of work shows how they were degraded and supposed to just accept it. Their faces show that they have been "beat up" by a country that supposedly is the free and equal land.
When I compare photos of slavery to photos of abolitionists and freedom, I notice that those feelings are never "gone" from prior slaves faces. It is instilled in their DNA almost that they were slaves. I truly love to see pictures of sincere abolitionists fighting for fair treatment of their brother men.
I interpret the photos of slavery regarding freedom to show the before versus after, good against evil, and representing crimes that should have never happened. Enslaved people should have never had to learn the lessons that they were exposed to.