wpa_gui only supplies two icon files: /usr/local/share/icons/hicolor/16x16/apps/wpa_gui.png /usr/local/share/icons/hicolor/scalable/apps/wpa_gui.svgz 16x16 icon is shown in the Applications menu. But tray doesn't show any icon for this app. Scalable icon should be used when nothing else is available for the required size.
This is totally up to the application, not our systray.