User update needed after updating to Mac OS X Yosemite
I got this message too and solved it easily:

Go to System Preferences;
Users and Groups;
Select the Admin User and click the padlock to make changes; enter the Admin's password;
Click on the picture of the Admin; the word 'edit' should appear;
Select Camera;
Take a new picture of yourself and save it;
Lock the padlock again;
Restart the computer.  You could probably take a different option instead of 'Camera' but I didn't try that.