Casualty results and vivid descriptions were made public as soon as August 8, 1945. The first in-depth discussion of the conditions on the ground, though, was John Hersey's Hiroshima, which was published as a full issue of The New Yorker in August 1946, and quickly published as a short book thereafter. Some photography of the damage was available in the 1940s. Detailed on-the-ground imagery of victims was generally not available until the 1950s, and much of it was not really circulated until the 1960s. The major governmental reports on Hiroshima and Nagasaki were not released until the 1950s.

So there have been varying degrees of information about the effects of the atomic bombs on Hiroshima and Nagasaki since very early on, though much of it was non-visual in nature early on. Certainly Americans were aware of destruction. Were they as aware as they could be, later? No. But Hersey's book pulled no punches. It's worth reading, even today — it is a short but devastating read, powerful in its strict description. It was a best-seller at the time, and so you can take it as a good example of what an educated, interested person would have been able to know about Hiroshima without recourse to specialist literature.

link to the documentary