Google provides a clear list of graphics assets are needed for promotion in their Play market, as well as sizes and requirements for launcher icons.
Maybe I'm missing something, but I can't find a similar list of graphics assets and icon requirements for iPhone apps in Apple's iTunes store.
Does such a list exist?
For the App Store you'll need one 1024px icon and at least one screenshot in retina solution (two if your app is universal - one for iPhone one for iPad)
In your app you'll need at least one icon for each device type you support (iPhone/iPad), but you probably should provide more. For different icon sizes an app should support please refer to this document.