Here's how to replace the pictures that tell you to "Please wait while Windows is shutting down" and that "It is now safe to turn off your computer" with your own designs:

1. Make backups of the files C:\LOGO.SYS, C:\WINDOWS\LOGOS.SYS and C:\WINDOWS\LOGOW.SYS in a temporary folder.
2. LOGO.SYS is the start logo, and
   LOGOW.SYS is the one that reads "Please wait while...," and
   LOGOS.SYS is the one that reads "It is now safe to...,".
3. Using graphics editor, tweak those bitmap images or edit any images of your choice.
4. The files are 256-color windows bitmaps (RGB-encoded, but not RGB color), 320 x 400. Since the aspect ratio (width / height) of these files are not standard 4:3, like most computer screens, the bitmaps will appear vertically elongated.
5. To make your new design conform to this aspect ratio, resize the bitmap to 534 x 400 while you're working on it. Make sure to resize them back to 320 x 400 when you're done. Save your changes, and rename the extensions of your new files back to LOGO.SYS, LOGOS.SYS and LOGOW.SYS.
6. Copy new files back into the appropriate folder.
7. To rempove the startup logo altogether, edit the file C:\MSDOS.SYS, and add the line LOGO=0 to the Options section.