Set the icons and thumbnail of the game
In the Project Manager there is an option for Icons and thumbnail.
When you click on Icons and thumbnail in the Project Manager, a window is opened where you can set icons for the game that will be used when it's exported as a Windows/macOS/Linux app or an Android/iOS mobile game. In this window you can also add a thumbnail for your game to be used if it's published on GDevelop's games hosting platform gd.games.
The quickest method is by clicking the button Generate Icons from a file. Click on it, choose a file, and then all icons will be generated at the correct size from the image you've chosen:
All icons must be in the .png format. Otherwise, the game cannot be built in .apk
Click on Apply when you've finished adding your icons and thumbnails to save them. You can also choose a file for each icon manually, but you must use the proper sizes for all icons to avoid any errors.