the preferences-desktop-keyboard icon given by xfce4-keyboard-settings
in its desktop file is not provided by its dependencies, resulting in it
having a "white square with red cross" default icon in the applications
menu if xfce4 is installed on a otherwise bare system. no icon is shown
in the general xfce4 settings application.

other similar applications' icons (like xfce4-settings'
preferences-system) are provided by tango, which is recommended by
xfce4.

i suggest ...

* xfce4-settings either chose among the icons provided by recommendations, or
* it recommend an icon theme that does include their icons, or
* the tango scheme be extended to include a preferences-desktop-keyboard
  icon.