For this week's resource I want to talk about EXIF data. This is information that is hidden inside a photographic file, (see page 187 in How To Puzzle Cache). In the real world this includes information such as the make and model of the camera, the shutter settings, and whether or not a flash was used. In the world of puzzles the EXIF data could be used to hide codes, coordinates, or other data.
Jeffrey's Exif Viewer is one of the easiest online viewers, you can paste in the URL of the photograph, or upload it directly. It even has a taskbar applet that places a link in your bookmark bar. Once you have a photo isolated to its own URL you can simply click that link to examine the EXIF directly.
UPDATE: Check out the second post for today.