iOS's file system: where are those files that are synced with the iPad via iTune? photos, videos, etc
Generally, the files are stored inside the application sandbox.
The application sandbox is a folder that stores the App and its associated files.
Every App has it's own application sandbox.
If you use file share in iTunes, the files are stored in a /Documents/ folder inside the application sandbox.
It is a bit tricky to access files that are stored outside the applications sandbox. 
Apple has a good and short explanation about files in it's iOS Application Programming Guide.
If you want to access on images from the PhotoLibrary, have a look at UIImagePickerControler.