With the introduction of Asset Catalog support for app icons for iOS 9, we no longer use the appicon.png and firstname.lastname@example.org images other than to resize them to generate missing images.
Furthermore, it build now requires appicon.png or email@example.com to be at least 180x180 pixels.
Since this is sort of awkward change of behavior, it would be better to introduce a new image called "DefaultIcon.png" that we use to generate all missing icons as well as the iTunesArtwork for iOS builds.
Initially this would be implemented for iOS. Android, Windows, and Mobile Web support would be implemented in different tickets.
Since the DefaultIcon.png should not be apart of a built app, it must not go in the Resources directory and thus shall be searched for in the project root directory.