I've got an iPhone app with icon file Icon.png.
This icon shows up properly when the app is on the phone itself, but it doesn't show up in the applications pane in iTunes.
What do I need to do to get it to show up properly?
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:
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).