Como você está usando o Win7 Enterprise, eu acho que eles são contas de domínio.
Lembro de ver um objeto de política de grupo para isso, então procurei e achei isso:
If you enable the domain GPO object Computer Settings->Administrative Templates->Control Panel->User Accounts->Apply the default logon picture to all users, the user account picture is set to an empty frame in both the logon screen AND the picture area of the Start Menu. This is even after replacing the user.bmp file in %programdata%\Microsoft\User Account Pictures with a different bitmap.
If you take a non-domain member system and enable the same local policy and replace user.bmp, the replaced user.bmp file appears as the account picture in the logon screen. However, once you've logged in, the start menu contains an empty frame account picture.