Every app needs a beautiful and memorable icon that attracts attention in the App Store and stands out on the Home screen. Your icon is the first opportunity to communicate, at a glance, your app’s purpose. It also appears throughout the system, such as in Settings and search results. You can manually provide each required image at each required resolution (12 images are required at the time of writing) or you can use the built-in icon generator to let Creo automatically creates all required icons for you. Please note that you should use your higher resolution image in this case.
Your macOS app icon should be recognizable, but not an exact copy of your iOS app icon. In particular, the macOS icon shouldn’t use the same rounded rectangle shape that the iOS icon uses. App Store, Maps, Notes, and Reminders provide icons for macOS and iOS that are recognizable, yet distinct from one another. Reexamine the way you use images and metaphors in your iOS app icon. For example, if the iOS app icon shows a tree inside the rectangle, consider using the tree itself for your macOS app icon.
macOS icons must be in the .icns file format and can be set in the App's inspector.