Avatars can be from 20 pixels to 90 pixels, and they don't have to be square. The avatar should be no larger than 15k.
To upload an avatar, to the "User Control Panel," which is the link just below the banner image that spans the top of the page.
Select "Profile" from the drop down under "Profile."
Next, select "Edit Avatar" from the right-side drop down.
Upload your avatar using the "Browse" button to locate it on your computer. Notice the file size restrictions. Press the "Submit" button to upload your avatar.
Once uploaded, your avatar will accompany your screen name in all forum posts.
You can change or remove your avatar at any time.