Unfortunately, NotificationCenter has limitations, and one of those
limitations is the artwork that gets shown. The only artwork that it
will display, is the app which sent the notification. In the case of
app's not running the Growl 2.0 framework, this will be Growl's icon.
This is not something we can do much about, it was Apple's decision to
implement things that way. Of course our own display styles will
continue to use the appropriate artwork.