The image must be very small (often 16×16, 32×32, or 72×40 depending on the device). The image usually needs to be:
Monochrome (1-bit)
Or converted to a byte array in code
The icon must be explicitly referenced in your game's source code (it won't auto-detect images). After adding the icon, you must recompile and reflash the game — uploading files alone won't change the icon.