From Apple documentation…
Note: If you are distributing your application to local users using ad-hoc distribution (instead of going through the App Store), your bundle should also include a 512 x 512 pixel version of your application icon in a file called
iTunesArtwork. This file provides the icon that iTunes displays when distributing your application.
The file can be a png or jpg, but must not have any file extension. Drop it in your Resources folder (see image) and build. When you drop it in iTunes now, you’ll see the icon.