Your application needs icons of appropriate sizes for different device screen
It will be enough for you to prepare only 1 icon.
Prepare 1 piece 512 pixel x 512 pixel icon.
After creating the icon, go to https://www.favicon-generator.org/ and create your icons using the online tool here.
Select the icon and select "Generate icons for Web, Android, Microsoft, and iOS (iPhone and iPad) Apps" and press the "Create Favicon" button.
On the page new page, you will see the link "Download the generated favicon"
Click the link and download the icons. Extract the icons and upload them to your website ımages folder ( via FTP or Media Manager )
Your icons are ready to use.
You can now install icons in App Icons.