Here is a post from James O'Neil's blog at Microsoft Technet that explains how it can be done.
A few of people have noticed that I’m
running Windows 7 with a customized
logon screen, and a couple of them
asked me if I used “logon studio”
which (as I understand it) rummages
round inside some of the image
resources buried in DLL files.
In Windows 7 we have provided a
registry key for OEMS to turn on
custom backgrounds it’s under
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
and the name is OEMBackground, you can
add it if it isn’t there – it’s a
DWORD 1 indicates use the custom
backgrounds and 0 means don’t. Beware,
if you change Windows themes this gets
reset – which implies the theme covers
the background as well.
Once the option is enabled you need to
create files in the
%windir%\system32\oobe\info\backgrounds
folder, the names are
BackgroundHeightxWidth.JPG and
BackgroundDefault.jpg so for this
laptop I have a default and
Background1920x1400.You can use this
make your corporate Machine all more
corporate or your personal machine
that bit more personal. According the
Windows 7 center where (I think) I
first saw the tip the file size need
to be under 256KB. If you want to
customize the original background you
find it’s named background.bmp in the
OOBE folder (oobe for those who don’t
know is Out Of Box Experience in
Microsoft speak).
