MAC: Installing your app icon - nwjs/nw.js GitHub Wiki

You will need a .icns file, size 1024x1024.

If you're not sure where to start, begin by creating a 1024x1024 .png file, you can convert it with a tool like png2icns.

Once your icon is ready, move it to YourApp.app/Contents/Resources/nw.icns, by overriding the existing one.