In the Summary section for your Xcode when you click on your project name under TARGETS, there are areas to select the splash images and icons - App Icons and Launch Images. And there are separate sections for both iPhone and iPad.
You right click on the space for the icon/image and select one that you have in your project directory and Xcode will copy and name it as appropriate. Make sure to pick images of the correct resolution and orientation for each icon/image.
This Apple guideline will show you the required and recommended images/icons. If you don't provide the required ones, your app will get rejected.
If your app is a landscape only one, like mine, it helps to make the Portrait images based on simply rotating your Landscape ones. That's what I do. So, even if the app hasn't finished changing orientation and the user is holding it in portrait, it looks correct.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master