You need to have the picture you want to use stored in your computer.
There is a maximum file size for avatars: 100 x100 pixels for a photo.
I believe that is the correct size.
At the top of the home page, click on profile
Click on Edit Options.
Scroll down to where is says avatars
Click on change avatar
Where it says Use Custom Avatar, mark that box yes
Go to where it says "Upload an avatar from your computer"
Click on browse
Find it on your computer
Once you find it on your computer, double click on it.
That should attach it.
Click on Submit modifications.