Hello Gabriel,
If you are using Business, Ultimate or Enterprise, then you can change this
via Group Policy:
Type gpedit.msc into the Start/Search box and press enter. Navigate to
Computer Configuration, expand to Administrative Templates, Control Panel,
User Accounts. In the right-hand pane, you should see the policy "Apply
the default user logon picture to all users".
Explanation of this policy:
This policy setting allows an administrator to standardize the logon
pictures for all users on a system to the default user picture. One
application for this policy setting is to standardize the logon pictures to
a company logo.
Note: The default user picture is stored at %PROGRAMDATA%\Microsoft\User
Account Pictures\user.bmp. The default guest picture is stored at
%PROGRAMDATA%\Microsoft\User Account Pictures\guest.bmp. If the default
pictures do not exist, an empty frame is displayed.
If you enable this policy setting, the default user logon picture will
display for all users on the system with no customization allowed.
If you disable or do not configure this policy setting, users will be able
to customize their logon pictures.
You can enable the policy, and remove the default user.bmp.