App icon

Recommendations

• First of all, take a look at the official guidelines by Apple and Google.
• It's a good practice to set the app icon as soon as possible. The designer is responsible for providing you with the app icon, but if he doesn't have the ability to do this, you could use a primary color and the app's logo in the center.
• On iOS it's easier to set all the icons in one place, but for Android, it's a bit unobvious. You should set square and circle app icons and keep them in the mipmap directory. You should also keep the icon for notifications in this folder.
• Remember - the app icon is the face of your application. Be attentive to the quality of the app icon.

Tools

For iOS, this nice tool might be used in order to generate app icons. For Android, if you don't have Android Studio or don't want to use it, then you could use this tool.

Examples

You may find the complete examples for Android and iOS.