Gita,
When you click on the link that SSG provided
http://garden.org/users/prefs/...
--do you see 'Managing Your Public Profile' at the top of the page?
--if you scroll all the way down to the bottom of the page, do you see a box that says 'your avatar photo'? Yours should have a picture of an acorn in it.
--do you see the box to the right of the acorn photo? It should say something like:
To change the photo, upload a new one below:
Choose File
--then you would click on 'choose file' and upload a photo from your computer
If you are not seeing this when you click on SSG's link and scroll to the bottom of the page, then we could find someone on the site for you who could provide more detailed technical assistance.