With Windows Phone 7 you can integrate your own application seamlessly into the Windows Phone built-in photo viewer. The key to this feature is the Extras.xml file.
You have to place a new xml file with the name Extras.xml to your Windows Phone 7 project root. Set the properties to Copy to Output Directory and select Copy always.
For the StorageFolder attribute you can choose your own name.
More informations you could find here: How to: Create a Photo Extras Application for Windows Phone