In Gnome3, when I've switched to overview mode (by pressing the "super" button), how do I navigate between thumbnail images of windows with my keyboard? How do I then select a window? Thx.
GNOME Shell currently (up to 3.4) does not support keyboard navigation natively. There already are a few bug reports filed, but for now try the Window Navigator or Window selector extensions. And of course, good old AltTab.
Upd: GNOME Shell 3.12 allows focusing the window thumbnails as any other widget, so they can be selected using keyboard and the Tab key.