In order to remove the user picture tile completely, you must edit the xml in the file authui.dll > UIFILE > 12400, 12401, 12402. Basically you have to delete any picture components in these three file, that should remove them entirely from the logon screen... be careful with this, I wouldn't recommend trying it unless you have a dual boot setup as you can really screw up the operation of your OS. I have been really glad to boot to vista and fix all this stuff on more than one occasion.