Hi ponjee,
I've just changed my avatar to this eagle I found on Clip Art. My first avatar was a photograph I took, but it was too small and couldn't be seen.
You have to go to the Gravatar website and register. You then select a picture from the Internet, your hard drive etc. The website then sizes the picture and checks it's suitability (nothing offensive). Once you confirm the picture, that becomes your avatar and will accompany any posting. IT took me about an hour of head scratching to initially set up my avatar, but it was easy to change.
Hope that helps.