The first newsreels on the damage at Hiroshima and Nagasaki were released in 1946. A Tale of Two Cities was released in 1946, and focused entirely on physical damage (not victims/casualties, except for the testimony of a Jesuit priest). (The film, it should be noted, contains many things said by the narrator that are not quite true.) Footage that included medical effects was generally not available until the 1950s.

We do not have, to my knowledge, much record of people's reactions to the newsreels, but they were part of a lot of other means to understand the new weapon. As for the feelings Americans had, they varied — many were happy that the bomb appeared to end the war, but many other Americans wondered wha kind of new world we were entering into, and what the next war would look like. It is a complex issue and there were many complex reactions. Spencer Weart's Nuclear Fear: A History of Images (Harvard University Press, 1988) and Paul Boyer's By the Bomb's Early Light: American Thought and Culture at the Dawn of the Atomic Age (University of North Carolina Press: 1994) are good places to start for the details. Neither these films nor any other single piece of information about atomic weapons was presented in a vacuum — there were many things going on and changing all at once, from the very serious to the kitschy.