The Palio di Siena is an ancient horse race around the square at the center of Siena. It only lasts 90 seconds, but the build up to it is about 3 hours long. This is the moment the horses set off down the first straight and the horse (and jockeys) start to break apart into thier race positions. It's the only moment in the race where all the riders are side-by-side and the of thousands of local spectators hearts are in their mouths.
This image feels a little closer to sports photography than art as such, at any other event it might be considered a little passé, but the location and the history make it hold it's own in my opinion.