On 28 September 2016 we have changed how the icon_url of the Pushpad API works.
Pushpad
Add push notifications to WordPress in a few minutes
In this tutorial we’ll see how to add push notifications to your WordPress blog. Notifications are real push notifications, meaning that they are delivered even when the user is not on your website. This way you can re-engage users when you publish new contents.
We are going to use the official Pushpad plugin.