Photographer David Bergman made the composite photo of the opening ceremony of the London 2012 Olympic “by shooting 425 individual photos in a grid pattern (25 across by 17 down) during a one-hour time period as the athletes entered the stadium. The final high-resolution image is 81,992 X 37,520 — more than three billion pixels.” [I would think the ceremony would be over before he had taken all the pictures.]
Visit the nbcolympics site to zoom and pan into the image. If you (or you know someone who) attended the opening ceremony, you can actually zoom into the picture and tag the face!