When you get a premium membership then you can post an avatar. Best way is to either go to Google.com, click on the images link, then type in what you would like for a picture.
Then you will more then likely have to resize the picture using any photo editing software, even the software that comes basic with your computer like Paint. Picture cannot be more then 100X100.
Then in the user account section, you click on your files, upload it then select it as your avatar =) presto chango, there you go.