Thomas Clarkson most likely wanted to model the cramped conditions that kidnapped slaves are forced to endure. By showing this picture of how they were crammed in the ship, it was more than likely a huge eye opener to some white slave owners. Because of how close together the slaves quarters were there was little room to sit or even use the bathroom. Many people died on the way as well, the living had no choice but to either try to pile them in the corner if they could or sit right on top of them for the remainder of the trip which could last months.
I believe Thomas Clarckson's reason for publishing this photo was to expose the horrors of slave trading. As seen in the photo, the conditions for slaves being transported are inhumane and dastardly. Maybe Clarkson believed that if people saw these photos, they would realize how horrible slavery is, and a movement to put an end to slavery would occur. Even though the reality was horrible, people still refused to give up their slaves at the time. Thomas Clarkson's published efforts were ultimately for naught.
