BTW--I did search on these forums and saw that the problem has occurred for others, generally as they were installing themes. The advice from cclear seems to be best:
-----following is quoted from a relevant post from cclear-----
I ran into the same problem... freaked me out for a...
Yesterday I installed a preference pane called Visage, that allows custom login backgrounds to be installed. Things seemed to work as advertised through several reboots, and I had a nice Happy Mac icon on my login screen. At some point today, I ran the Disk Utility and repaired permissions...