Icons are automatically included to the application during the cordova build process
These icons / splash screens are placed in src/res for each platform
You may replace these icons with your own.
You can generate your own icons by using the script within src/res/icons.sh
This script uses svg files as an input, and generates all icons and splash screens using linux command line tools (see source code of the script for more info).
After the script is completed, all icons will be replaced within src/res and you can rebuild the application which will now include your new icons.
NOTE The script does not guarantee that the icons will be perfect. Please make sure about each of the files that are generated. The script is only provided as a shortcut to quickly create icons in the required sizes, normally you would have a marketing department source you icons in the required sizes.