Editing A System32 File?

Its pretty nice, except that when I get to the login page for windows, it shows a picture of a finger print instead of my user pic. i found out that the image for the fingerprint scanner is part of a file called "BioCredProv.dll" in the system32 folder. So I made a cop of it, edited the copy with a dll editor so that the bitmap image is that of my user account, but now I can't replace the file in the system 32 folder. I've tried deleting the original and replacing it with the edited one, I've added 1 to the end of the edited file's name so it is "BioCredProv1.dll" and then attempted to change the registry to use this new file (but it gives me an error saying "Cannot edit: Error writing the value's new contents." I know it can be changed as I've seen another forum where two people gave two ways to change it (one using an ubuntu linux system to change the system32 file which I don't know how to do, and the other by creating his own new registry file, which I once again don't know how to do.)