1. A system comprising: a data processing apparatus; and a computer storage medium encoded with a computer program, the program comprising instructions that when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving, from a first user, first image data corresponding to a first image; receiving, from a second user, second image data corresponding to a second image; comparing the first image data and the second image data; and determining that the first image and the second image each correspond to an event based on the comparing, the event being coincident with respect to the first image and the second image.