There are two pictures from the Vietnam War that are perhaps as iconic as any and which had an impact on the American perception of the war.
The first of these was taken in 1972. It is the photo of a young girl running naked down a road and apparently screaming and/or crying. She has been badly burned in a napalm attack by South Vietnamese forces. This photo is important because it helped to push American opinion even further against the war. By this time, Americans were already pretty skeptical about the war, but this picture helped to intensify those attitudes. It was a picture that showed very graphically how innocent people were being harmed terribly by the war.
The second of the pictures was taken in 1968. It is the photo of a South Vietnamese police official killing a handcuffed Vietcong prisoner by shooting him in the head with a pistol. This picture helped to turn Americans against the war because it appears to show the arbitrary nature of justice as meted out by our South Vietnamese allies. After all, this was neither a shooting during combat nor an execution that happened after a trial.
Both of these photos helped to turn Americans against the war.