Hey Paul,
The most difficult part is creating the avatar, because it must be under 6kb in size. You will need to edit your picture in whatever photo editing software you have. The exact steps will differ depending on the software, but here's the general idea.
(a) Crop the picture so it's square. This is not required, but will look best and maximize the pic.
(b) Save the picture as an 80x80 pixel .jpg and medium quality. If it is not square, use 80 for the widest dimension.
(c) Check the size of the file and make sure it's under 6 kb.
(d) If necessary, resave it at a lower quality. This decreases the file size. Repeat (c) and (d) if necessary.
When you have your avatar as a little .jpg, follow these steps on this website:
(1) Click "User Control Panel" near the top/center of the page.
(2) Click "Profile" on the left side of the page.
(3) Click "Edit Avatar" on the left side of the page.
(4) Click the "Browse" button next to "Upload from your machine:"
-- Find the .jpg file on your machine and click "Open", "Select", or something similar. It may vary on different computers.
(5) Click "Submit"
-- You should get a message saying your profile has been updated.
-- If it doesn't like the .jpg you will get an error message instead, but so long as it's under 6 kb in size you should be OK.
For almost everyone you can ignore "Upload from a URL:". If you know you have a tiny .jpg out on the web and know the URL then you probably don't need any help
.