Since yesterday's update my iMac login screen has lost it's wallpaper. The screen is black, but for the various icons. The keyboard doesn't work on first keystroke (hasn't done since I encrypted the HD) so I have to press the left/right arrows anything from a couple to several times before either of the potential login names become highlighted. This was vaguely annoying when there was a blurred wallpaper behind the login screen as I could at least see when a name was highlighted. Now I can't see the highlight at all. This behaviour is not duplicated on my Mac Mini or MacBook Pro, just on the iMac. I've tried changing the wallpaper and rebooting, but it seems to make no difference.