When an app try to send a notification while Xfce4-notifyd is running, and it specifies icon as a path to an SVGZ file, Xfce4-notifyd doesn't display that icon. Though, if uncompressed SVG is passed, it works good. On a side note, GNOME is affected by a similar issue as well, see https://bugzilla.gnome.org/show_bug.cgi?id=655403. I consider it a bug since SVGZ is part of the SVG specification.
This needs to be fixed in gdk-pixbuf.