how do you change the logon background image?

I have tried TweakUILogonWallpaper, and I have tried using ResHack, and I
have tried using the few registry hacks that are out there, but not one of
them works! Whatever changes I make don't seem to take, and it goes back to
default. I think maybe it's because Windows File Protection is stopping any
changes, though most of the tweaks I've read out there don't seem to mention
WFP at all. And yes I am the administrator of this pc, I have full access.

Any ideas? All I want to do is change the logon background image...seems
simple enough.

Tim