Not sure if you've found your answer yet, but this information might help.
In Vista and Win7, the pictures are stored here:
%PROGRAMDATA%\Microsoft\User Account Pictures\
[typically C:\ProgramData\Microsoft\User Account Pictures\]
The pictures are 128x128 BMP files.
There are a couple of related Microsoft posts here:
On 1/29/2011 9:47 AM, Larry Kost wrote:
> Does anyone out there know how to change a user account picture without going through the
> Control Panel/User accounts/ etc? Can it be done in the registry or User's Profile?