As you are using Win7 Enterprise, I guess they are domain accounts.
I remember seeing a group policy object for this, so I searched and found this:
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.
Link