Ionic Push Small Icon

I want to show an icon on my Phone when a notification is sent. I managed to show the big Icon which is working fine. But the small icon in the notification bar is a white square. I have read several guides regarding the requrements they should all be met. My code currently looks like this, maybe I am referencing the icon in a wrong way, since the plugin-documentation states, that the image is supposed to be handled as a resource, but I can only generate resources under Ionic for the splash and the icon:

public static PUSH_OPTIONS: PushOptions = {
        android: {
            senderID: 'id',
            icon: 'assets/img/notification.png'
        ios: {
            alert: 'true',
            badge: true,
            sound: 'false'
        windows: {}