Users store their images in their own cloud storage (Dropbox or Google Drive), and safely share those images with invited others. Facebook users can be invited.
Conversations about the images are also stored in user cloud storage. And users that can access the images can access the discussion threads.
On the Apple App Store as Neebla. This app uses the SyncServerII backend.
Straight out of the box, this project will not build cleanly. See these instructions.