If you want icons show up on iTunes, you need to add at the root of the Xcode project png files with these names and sizes:
- iTunesArtwork (512x512)
- iTunesArtwork@2x (1024x1024)
(yep, no file extension)
If you don't, you get this on iTunes: http://imgur.com/6hyAxLU
My suggestion would be to get icons of those sizes from config.xml and add these files on the root of the project automatically, as a part of the icons processing.