When spectators with cameras were fleeing, they headed towards the madness of the explosion. Tlumacki took his iconic picture just 15 seconds after the first explosion.
Think about this for a moment. At the finish line, there must have been hundreds of cameras. You would think everyone was taking pictures and recording videos. Twitter as a publishing platform is primed and ready for stills and video. It's a world where everyone has a camera. But I saw very few images from the general public.