As with the existing generic console app logo, I'd suggest a generic GUI app logo as well; it seems like the most obvious missing choice. For the image, how about something like scalable/apps/preferences-system-windows.svg from gnome-icon-theme, found at http://svn.gnome.org/svn/gnome-icon-theme/trunk/scalable/apps/preferences-system-windows.svg ?
Hmmm, come to think of it, the GPL that applies to this image actually does come into play, since you'll distribute a PNG rather than the original SVG source.
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