Logon wallpaper customization

Hello,

I'm trying to change the background of the logon environment in the
windows xp pro with something funny like a bmp image or jpg image. I'm
not using the welcome screen, so i get only one dialog asking for the
user and password, and the background is totally empty filled with the
well known blue.

I already try to seek how this can be done and if is possible of
course, and i already try to change the following regestry key:
Hive: HKEY_USERS
Key: Default\Control Panel\Desktop
Name: Wallpaper
Data Type: REG_SZ
Value:Path to the bitmap

And the only result that i get is quickly after the logon the image
appear and after the logoff the image appear also... but i want to
display the image before of the user authenticates in the s.o.

Is this possible? and is possible without using any tool only changing
some settings in the regestry? And by the way is possible to change
the logon environment with a custom image with the "Use the welcome
screen" option turned on?

Thanks
Nuno