read this first carefully
*www.thinkdigit.com/forum/showthread.php?t=31112
and remeber reduce the wallpaper size to 640x480 and save it in bmp format then only u will be able to replace that resource(Bitmap>1000>1033)
if you want only wallpaper and no top and bottom darkblue strip then u have to edit the UIFILE>1000>1033 also in loginui.exe
WARNING: Remeber if you will done anything wrong then your system will not boot so keep a winxp bootable disc with you for emergency