Xcode 9 - Add an App Store icon

If you are using Xcode 10 or earlier in MAC

Same icon use in appstoreconnect and another one xcode General->App icon and launch image->app icon source->add image No transparent nor contain an alpha channel

If you're using Xcode 9 or later, you should use your Assets catalog (See image below) to provide an App Store Icon for your app like below and there is no other way. This way it would be easy to prep. for App Store release. You can mostly use Xcode to configure things, instead of going to iTunes/App Store Connect.

If you are using Xcode 8.3.3 or earlier, you must use iTunes Connect to upload your App Store icon

As of now (September 2017) you cannot push apps built using Xcode 9 since it is still in beta. If you want you can wait for another day or two until Xcode 9 GM build gets released. Your build will fail in the upload process if you are using Xcode 9 without adding the App Store icon resource.

For more info read this.