The principle is the same, upload an image (png, gif, jpg) and this free on-line tool creates the Google map shadow and suggests an array of x,y pixel coordinates that defines the clickable area of your custom icon.
Transparency of the original image is preserved. You get to see your image as a marker on a map and are presented with a zipped package for download that contains the generated images and some sample code.
An easy way to make an icon shadow for Google Maps API v3
Feedback and comments are welcome! I would love to see some sites where your generated custom markers are in action.
Update 20/02/2011: The zip archive created should now extract on Mac systems as intended. Many thanks to all of you that highlighted the situation to me in the first place.