Why can't I see thumbnails in the finder? Most of the instructions I've found online (this, for example) say all I have to do is select "show icon preview" in the finder view settings.  But that doesn't seem to be working.  What's the problem here?

This is OSX 10.8.5.
This just happened to me and it was fixed by relaunching Finder:


Alt + Right Click Finder in the Dock > Relaunch
Cmd+Alt+Esc > select Finder > Relaunch