Design an appropriate bitmap and place it in your NT directory as YourLogoName.BMP. Use 8.3 naming convention.

Edit: HKEY_USERS \.DEFAULT\Control Panel\Desktop

Double click or Add Value REG_SZ of Wallpaper and set it to the full path to YourLogoName.BMP.
Double click or Add Value REG_SZ of TileWallpaper. 0 is Don't tile, 1 is tile.
Double click or Add Value REG_SZ of WallpaperStyle. 0 is normal, 2 is stretch to fill the screen (which is mutually exclusive with TileWallpaper = 1).
If you use a normal, not tiled logo, you can position it by adding the following REG_SZ values:
WallpaperOriginX set to the number of pixels from the left hand edge of the screen.
WallpaperOriginY set to the number of pixels from the top of the screen

Reboot to view your handywork. See Tip 205.

NOTE: If you set WallpaperOriginX and WallpaperOriginY to 0, it centers the wallpaper. If you meant top left corner, set both to 1.