Windows 7 User Logon Picture Changed On Its Own?

I'm fairly technical and was a network and tech Director for a few years so have some familiarity with prior Windows versions but Win 7 is a bit new to me. I went to login today, and my login picture had been changed. Looking in Windows Event Viewer under Security, I can see it occurred at 12:25 in the morning. I was on another computer at that time, and running a utility from Microsoft on the PC in question that just collects system information.I was trying to figure out why it changed and reviewing the event logs. I don't see the reason, only that an account change occurred. I replicated it by changing my picture again to another one, and the same system event occurred confirming its identity.So basically, I had a flower as the login picture. On its own, it changed to a fish. I didn't make the change. I wasn't even using this machine at that time; however a Windows utility from MS was running -- but -- I reran that same utility tonight (it's one that just reports system usage) and it did not change the picture so I believe that is coincidence. why would Windows 7 change the default logon picture for a user on its own to something else? To help with information the utility that MS had me run was the Microsoft Product Support Reports.

There are also multiple instances on this computer, the one where the picture changed on its own, of another PC that was running Windows 2000 logging into it as a SYSTEM, and then logging back out, then in and out continuously but not in a pattern that would make me think it was a process. My other machines on the network do not have that issue in the log file.I checked the McAfee Firewall, and it was enabled. It has been catching attempts to login to VNC but they've been denied. VNC is also disabled.