Like they have said. Sign up with gravatar.com using the email address you are using now
Find an image and it's URL address online, like this one: Bloody, Bloody Andrew Jackson.
Click on the An Image on the Internet button and paste that address in. They might have you crop the image down to a square so pick the best part.
Of course, if you have an image on youy harddrive that you want to use then use the My computer's hard drive button.