This is a really easy one, but since I had to crawl across the gravatar site to find out how to do it, here is a quick introduction to get the URL for your gravatar. No thrills and frills, but it does what most of us will need.
First, you need to create a gravatar account, if you don't already have one. Gravatar is a site that aggregates avatar images for the different social networking sites (like Facebook, MySpace, etc.) and blogs (like Blogger). It works by storing an image of you (of your choice) and associating it with your email address.