First things first…
The default user account picture I’m referring too it’s the one that a new user get’s when it’s created.
This pictures appears on the logon screen and also on your start menu.
Here’s an example:
This picture must be:
Size: 128x128 pixels
If you have another picture and want it to be the default picture for every new user, just copy it to:
c:\ProgramData\Microsoft\User Account Pictures\user.bmp
The picture must have exactly the same name (user.bmp) so you’ll need to overwrite the default one.