Jenny,
go to "My Controls" which is located up at the top of the page, right
under "Classifieds".
Once there, over on the Left-hand side of that page, you'll see a category
'Personal Profile', within that category see "Edit Avatar".
Change your Avatar from there, including editing the Size of it.
Weird that I just saw your post here, I just finished changing my avatar from a pic
of my car, to one of the 'ideas' for a RoadGlue.com sticker.... Rollbar & Spoiler included!
Good Luck