Nowadays every photo taken with a mobile phone or digital camera contains metadata. This metadata, called Exif data, contains information about your camera, like brand and model name, the dimensions of the image and - when available - also GPS coordinates which reveal the location where the picture was taken.
File NameExample San Francisco.JPG
Creation Date2012-04-02 14:00:17
File Size358,767 bytes (350.4 KB)
Dimensions1024 x 768 pixels
GPS Coordinates 37.78983, -122.40367