I think that would still strongly depends on the kind of technology you have. I'm assuming about WWI-level of slow-exposure hand-cranked cameras, which would most likely give you a powerful propaganda tool, but not yet a reliable record of events. With both sides pre-war having vocal partisans, I assume the biggest question at the beginning is whether the industry is distributed equitably. If it's mostly an artisanal thing, depending perhaps on high-tech imports from Europe, you could see motion pictures from both sides in the run-up to the war. The abolitionists have the more compelling story to tell, but much will depend on who tells it better. With an industry concentration in the north (New York, most likely), you'd have a crushing weight of quantity going against the south. It might lead to a distrust of motion pictures on principle (they're all northern).
Of course, you would also have local authorities policing 'decency', so a lot of northern propaganda would never be shown in the deep south. But movie versions of Uncle Tom's Cabin or the lives of Sojourner Truth and Frederick Douglass might have an impact in the border states. It's hard to make the Southron case in film without the redemption narrative. I'm not sure movies of a slave insurerction would pass 'decency', no matter how much of a cautionary tale they were. These things are a two-edged sword, even illiterate blacks could watch a movie easily enough. John Brown, of course, would become the bogeyman of hundreds of cheap productions.
With the war on, both sides are likely to produce films that heap opprobrium on the other, discrediting moving images further as pure propaganda. THere's going to be films of Southern 'gentlemen' abducting free blacks at gunpoint, ravaging the countryside, chasing fugitives with bloodhounds and massacring prisoners, and movies of leering Yankee invaders pursuing Southern belles, irresponsily freeing savage blacks to stalk Southern womanhood, destroy homes and farms, and generally offend any sense of honour. Later on, you'd get images of the Shenandoah Valley, Andersonville and Fort Pillow, of bushwhackers' victims, looting and raping coloured infantry and pillaging rebel horse. THis would be strictly for consumption on your own side, and with the blockade in place, I suspect the northern narrative would export more successfully. AS to authenticity, assume everything is staged until you have proof to the contrary.
The biggest opportunity for changing history as we know it (assuming the war basically still happens as per OTL) will be in the northern elections. The peace democrats will certainly use reels of the horrors of war extensively.
THe later Western campaigns are unklikely to produce much controversy. Only one side holds the tools of producing mioving pictures, and even those who generally did not support slaughtering Indians considered them savages in need of imüprovement. You'd see lurtid reenactments of heroic stands by cavalry against the warriors of picturesquely named chuefs and bucolic scenes of hapopy INdians on reservations, gratefully accepting government largesses from their agent.