1) Click on Profile above.
2) At the bottom of your profile click "Browse" to the right of the field which mentions uploading an avatar from your PC.
3) Select an image you've saved somewhere on your PC which meets the following criteria:
Only one image can be displayed at a time, its width can be no greater than 80 pixels, the height no greater than 80 pixels, and the file size no more than 74 KB.
You might have to resize your image first.