Well - I wouldn't be as bland as that - it must be at least say 20 secs out of synch - and it may have looked in synch to those who chose to publish it! You can't make that mistake accidentally!
Take a look at Mishelle's links above. The fascinating fact is that the photographs of Hiroshima after the bombing look almost identical to the aftermath of the fire bombing of Tokyo. In particular, there doesn't seem to be a crater at ground zero.
Well if the scale was faked, could you definitely tell? Remember also that in the early days, this was an effort to end the war with Japan, you could consume a hell of a lot of TNT to achieve that! The mushroom shape is not exclusive to atomic weapons.
I am not that keen personally on exploring conspiracy theories - but I'll bet this one has some legs.