That was the tutorial I read but its a little complicated.Boiled down to essentials...
Create image(about 500x500)with or without shadow,above a layer of perfect black.Switch to channels for your image,add a layer(it will appear black in the thumbnails).That is your alpha channel.
Switch back to layers and contol click your image layer.Go back to your alpha channel and fill the slection with perfect white.(black will be transparent and the white areas will be what shows)
Save the image as Bitmap_128 (32bit bitmap)Paste it in the folder for your new logon.
If you need a logon file to work from you can use this one if you like...all you have to do is replace the images with your own and edit the UIFile in notepad to change text colors and element positions.
There is a photoshop file too if you want to see how the channels look.
WWW Link