Bigger icon (96x96 and scalable) installed & gnome-shell compatibility
Gnome-shell (see attached screenshot) uses for Alt-TAB switching menu large icons (96x96 or scalable). However, when they are not available, small icon is used and the result is awful.
For more issues how to make a program gnome-shell-compatible see http://live.gnome.org/GnomeShell/ApplicationBased