1.There are two files in the \Windows directory, both are 320 x 400 bitmaps
2.LOGOW.SYS is the "Windows is shutting down" screen
3.LOGOS.SYS is the "You may now shut...." screen
4.You can edit these files or create your own, they just have to be 320x400 bitmaps

If you have 640x480 images, here's how to make them 320x400 and keep their normal appearance when they are displayed at start up or shutdown:
1.Open the desired image in Paint
2.Choose Stretch/Skew under Image
3.Select Horizontal and change the percentage to 50
4.Choose OK
5.Choose Stretch/Skew again
6.Select Vertical and set it to 104
7.Choose OK
8.Choose Stretch/Skew again
9.Select Vertical again and set it to 80
10.Choose OK
11.Choose Attributes under Image and there it is - a 320x400 bitmap

Remember to save it with the .SYS extension

To create your custom Start-up Screen
Create a file as described above, and name it Logo.sys. Place this file in the Root directory (e.g. C:\)