The Extras menu of the built-in picture viewer app allows you to add a link to your app so that the user can quickly launch your app from within the picture viewer. This effectively allows you to extend the picture viewer with your own app.
An Extras application may be used to enhance an image’s color, crop an image, convert an image to monochrome, or even read a photo’s Exchangeable image file format (EXIF) information to display where the photo was taken on a map.
For an example of an app that reads EXIF data, see the Photo Location app on the Windows Phone Marketplace. (Full disclosure: The app is written by my wife.)
When users tap the three dots on the application bar in the picture viewer, they see ...