The most fearsome looking, and probably the most famous picture of a mushroom cloud is this one:
Notice that the lighting appears to be from directly overhead; where the sun would be at noon. This presents a problem since Hiroshima was bombed at 8:16 AM. The sun should be much lower. To remedy this, the officials claim that the above is a picture of Nagasaki, which was bombed at 11:02 AM. And the picture below is of Hiroshima. Apparently, not everyone got the word because the above picture is also claimed by many to be the one of Hiroshima.
See Star Bulletin
Hiroshima and Nagasaki Remembered
Some say the picture is Nagasaki, some Hiroshima, but most don't attributed to either. It's just a generic mushroom cloud.
This picture doesn't really solve the lighting problem. It still appears that the lighting is coming from a steep angle. Notice the shadow at the bottom of the white cloud. It appears to be very short.
Notice also the grainy appearance in the background of both pictures. Photography in 1945 was much better than this. Notice that at the very bottom of the white cloud, in the bottom picture, there is a small area where the dark smoke, is very plain and sharp, but around that, it is blurry.
This second picture just doesn't have that good, mushroom look to it, like the first picture. That might be why the first picture seems more popular, and turns up more often in internet searches for 'mushroom cloud'.
