What algorithm did Windows XP use to choose your initial user picture?

Windows XP selects the initial user picture using the RtlRandomEx generator seeded with the current GetTickCount() value and a one-pass reservoir-sampling algorithm (k = 1). The code iterates through the files in %ALLUSERSPROFILE%\Default Pictures, updating a winner with probability 1/count for each item, and stops after examining 100 pictures to prevent excessive processing.