.setSmallIcon() 에 넣으시면 되구요.
저의 경우에는 54x54 사이즈에 흰색과 투명색만 가지고 만들었습니다.
NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(this)
.setLargeIcon(BitmapFactory.decodeResource(getResources(), R.drawable.icon_launcher))
.setSmallIcon(R.drawable.icon_status)
.setTicker(getString(R.string.app_name))
.setContentTitle(getString(R.string.app_name))
.setContentText(sMessage)
.setStyle(new NotificationCompat.BigTextStyle().bigText(sMessage))
.setColor(getResources().getColor(R.color.primary))
.setDefaults(Notification.DEFAULT_LIGHTS | Notification.DEFAULT_VIBRATE | Notification.DEFAULT_SOUND)
.setAutoCancel(true);