Changing the screen saver authenticate background

Just let me preface, I already know how to change the login screen background -- that's not what I want to do.

When I wake my macbook from sleep or screensaver, it asks me to authenticate (as it should). But the background of the authenticate screen is a boring blank black screen. Is there some plist property I can tweak to get a pretty picture (like my desktop/login backgrounds) to display there?

I've been googling an answer for a day or two now and can't find anything.

Thanks!

It is ~Library/Preferences/com.apple.loginwindow.plist

This link might be helpful to you:
Peachpit: Make Your Own Start-up Screens in Mac OS X > Change the plist, change the Message Shown