When deploying an application using the ad hoc distribution “style”, the developer has to supply the image shown in the installed applications tab of iTunes. This file has to be a 512 by 512 PNG or JPEG image called iTunesArtwork. According to the documentation the file has to be extension-less, although when the artwork file (without extension) is dragged in the Resources group as documented, Xcode will add the extension again!
You have to manually delete the extension (again) for it to work and show up in iTunes, this last and vital step isn’t documented. Without the iTunesArtwork, iTunes will show a generic image for your application.