We now have the welcome and user-rights notifications defaulting to the generic notification icon, but we want to override these on wikipedia projects.
Within echo each notification has a name of the icon that should be displayed. $wgEchoNotificationIcons is a map from these names to the data necesary to load them. Both the welcome and user-rights notifications use an icon named 'site'.
CommonSettings.php already has:
$wgEchoNotificationIcons['site']['url'] = $wmgEchoSiteNotificationIconUrl;
InitializeSettings.php should be adjusted to something like:
'wmgEchoSiteNotificationUrl' => array( 'default' => false, 'wikipedia' => 'https://commons.wikipedia.org/....', 'wikibooks' => 'https://....', ),