How can I make my ad hoc iPhone application’s icon show up in iTunes?

The cleanest way to do this is described in the official Apple documentation, in a section called Publishing Applications for Testing. Below is the exact instructions given to you on that page:

The iTunes artwork your testers see should be your application’s icon. This artwork must be a 512 x 512 JPEG or PNG file named iTunesArtwork. Note that the file must not have an extension.

After generating the file of your application’s icon, follow these steps to add it to your application:

1. Open your project in Xcode.
2. In the Groups & Files list, select the Resources group.
3. Choose Project > Add to Project, navigate to your iTunesArtwork file, and click Add.
4. In the dialog that appears, select the ”Copy items” option and click Add.

Note that the PNG or JPEG file is just ‘iTunesArtwork’, with no suffix.

If you try to copy the file into the application bundle after you have built it, it will break the app signing, and you will get a verification error when trying to sync it to your device. Ensure that the artwork file is included in the “Copy Bundle Resources” folder, within your project’s target in XCode (step 4, above).