To make Application Icons for both iOS and Android, use https://appicon.co/
The advantage of appicon.co is that after adding the base icon image, it generates the set of standard icons, and automatically downloads them to a ZIP file named: AppIcons.
Extract the zip archive, it contains
- appstore.png: For Apple AppStore
- playstore.png: For Google playstore
- android/: A folder of standard Icons for Android
- Assets.xcassets: A folder of standard Icons for iOS