Here's what I have so far:
It does alerts too, I just didn't have any and it's easier to mark a message unread than it is to forge an alert.
When you visit a site that uses XenForo and you're logged in, it adds that site to the list of sites to check updates for. It checks every 5 minutes. If you get an alert or a new message then it will pop up this notification.
It uses your open graph logo for that image.
You can install it from here: https://chrome.google.com/webstore/detail/forum-alerts/leiehnemnmkljaaahpadjodmghkhlhjj