First, design an appropriate bitmap and save it in the C:\Documents and Settings\All Users\Application Data directory as LogoName.BMP. Use the 8.3 naming convention.
Since the Application Data directory is a hidden directory, you may need to unhide it by opening Windows Explorer and going to Tools > Folder Options, click on the View tab and check the Show hidden files and folders option. Click on Apply and then on OK.
Next, open the registry editor by going to Start > Run, typing regedit and pressing [Enter]. Navigate to the key HKEY_USERS\.DEFAULT\Control Panel\Desktop. In the right pane, double-click on the string called Wallpaper. Under Value Data enter the full path to your bitmap image - for example C:\Documents and Settings\All Users\Application Data\LogoName. BMP.
Now double-click on the string calle TileWallpaper and change the value according to your preference - the value '0' will not tile the image, while the value '1' will tile it. Next, double-click on the string called Wallpaper Style and change the value according to your choice; here the value '0' is normal, while the value '2' will stretch the image to fit the screen.
If you don't want to tile or stretch your wallpaper, and would rather choose its position on the screen, you will need to create two new data strings in this same registry directory. When at HKEY_USERS\.DEFAULT\Control Panel\Desktop, right-click on the right pane and select New > String Value. Name the string WallpaperOriginX and set the value to the number of pixels from the left edge of the screen that you want the picture to be located. Now, create another String Value, call it WallpaperOriginY and set its value to the number of pixels from the top of the screen.
Close the Registry Editor, and reboot you machine to see your handiwork.