I've updated the example, you could use the imageURL expression from the canvas plugin or reference project assets directly for use as icons.
I'd suggest using images of 128x128 in dimension to accommodate for larger notifications. (i.e nwjs - win 8/8.1).
To enable notifications on that platform, you may have noticed a shortcut to nw is added to the start menu ('projectName.lnk'); If the project name is empty it'll be called node-webkit.lnk ...
To remove it, right click the tile, open file location and delete the lnk file.
On older windows platforms, a system tray notification is displayed instead.
In this case notifications aren't displayed concurrently and the newest takes priority.
For "Firefox OS <1.1 you'll want to use an externally hosted version"... of the icon.
"[T]he notification is displayed immediately with the icon missing" until it's loaded.
— Thanks for the mention in the credits, it's optional but appreciated none the less.
Hope this helps.