The Launch and Icons Screen
The top panes (1) allow you to add splash screen images. Include PNG images, ideally of your centered app icon on a transparent background. Choose whether or not your app supports dark mode, and if it does then upload appropriate images for Dark Mode launch. For a discussion on what you should use here and why, see this lesson.
The bottom panel (2) is for your app icons. If you hover over the three dots for each selection, it will tell you what size you need. For maximum compatibility, you should add an image for all these options.
The Prerendered Icon checkbox is a legacy setting, you should ignore it.
Click the dots to choose your icons.