Ok, so the abolitionist movement had been building up steam for quite a while. Of course, this was happening up north with people that didn’t have a financial stake in slavery; it was easy for them to give up that which didn’t benefit them directly. Which meant it was also a fantastic rallying cry. It benefited from being morally good and could be used to paint the other side as bad. Again, wrong and wrong in historical context were two different things.
Also, this either was or became a position that Lincoln felt strongly about. I would be surprised if photography didn’t play a role in bringing about emancipation. As the world modernized, things like the cotton gin and steam engine/railroad heralded in very different times; slavery didn’t have a place anymore.
:)