Apps created outside of Apple and in the App Store have no way to change what graphic is shown when the app is launching. You can display a static image (Default.png), but that cannot be dynamically modified later.
This is irrelevant. The bug happens specifically while using Facetime, an Apple app, so it might be feasible for it to set the last image as the launch image. Also, Apple is notorious for using undocumented APIs in their own apps.