notification icon does not seem to be displaying

[Chris-CWZ]

I've had no problems setting up notifications, everything is sending properly with the proper subject and body, it's just the icon is still the default bell icon despite the fact that i've set it to be a different one explicitly. Below is my code to generate the OneSignal Message.

public function toOneSignal($notifiable){
        return OneSignalMessage::create()
            ->subject('Your registration has been approved')
            ->body('Login now!')
            ->icon(url('images/push_icon.png'));
    }

The icon should be pulled from the images folder under the 'public' folder in my Laravel app, with the name of 'push_icon.png'. However, the icon is still wrong when I send out the push notifications. Am I doing something wrongly? Thanks for your attention and help.

[metallurgical]

@Chris-CWZ have you make it work display an image?