Clearly the USA was rather indifferent towards the Nazi regime earlier on. To what extent did pictures and media play a role in that reversal? The images from the Holocaust are pretty abundant now. Where they displayed prominently in the 1940s?
They weren't. At least, not pictures on their own. The media only after other events took their cause (Pearl Harbor attack).
The real Holocaust and all its horrible extremities were much of a surprise to the Allies. The Nazis (to differentiate them from ordinary Germans) knew very well what they were doing was not something they should be proud about. The real commands at the very top were given verbally. Not written. On a lower level (Wannsee and below) everything was minutely administered. Which was a great help at Nuremberg.
To answer your question: photography wasn't responsible for the shift. It couldn't be: only very few people aside from the perpetrators knew what was going on. Walking around with a camera near a concentration camp was a good reason to get locked up in it. The people living near concentration camps knew it was pretty bad, but didn't ask any questions. It wasn't wise to ask questions.
So generally speaking, not that many people knew about the atrocities. It was known to be pretty bad, but how bad it really was, was not widely known.
What was responsible for the attitude shift? First of all the Japanese attack on Pearl Harbor. The America First Committee disappeared overnight, literally. Second would be the British propaganda efforts. Britain put a lot of work into changing the public opinion of the USA.