In the "My Stuff" menu on the left, click on "My Profile". As you are looking at your profile, there should be a yellow box right above the "My Stuff" menu. click on the "Upload Profile Picture" link inside the box.
From here on, its pretty straight forward. Click on the browse button to browse to the picture you want as your avatar, and then click on "Add Profile Picture". Depending on the size of the picture, the uploading of the picture might take a while, so just wait patiently.
When the picture has been uploaded successfully, the page will be refreshed, and you will see your picture at the bottom of the page. click on the round circle below the picture you want as your avatar to select it and click on "Select Profile Picture".
Voila, all is set. You can have up to 10 different avatars to choose from at one time. To delete an avatar, select the picture by clicking one the circle below it, and click on the "Delete profile Picture" button.