3rd-party apps can display a single static image (default.png) on boot. It is common for this image to be a screenshot of the UI without any content (e.g. nav bar, toolbars, but no buttons or data) in order to give the impression of a faster/more-progressive loading process.
I don't know how it is implemented under the hood, but Facetime on iOS does not appear to be a separate app (from the user perspective, at least). Someone who has jailbroken their phone and inspected the filesystem may be able to speak to this point more directly.
Fast app switching, depending on memory availability, does sometimes freeze an app and save state to stable storage. It's possible that this is a source of the rogue image data. A quick test with Instagram suggests that the camera is disabled when an app is "backgrounded." Perhaps Apple's built-in apps (e.g. Phone, Facetime) behave differently.