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 Googleplex.JPG
Creation Date2012-04-01 18:11:27
File Size274,681 bytes (268.2 KB)
Dimensions1024 x 768 pixels
GPS Coordinates 37.42183, -122.08433