1) Get the image you want for your avatar.
2) Make a small copy - no more than 100x100 pixels, no more than a 2kb file size
3) Either save it off somewhere on the web you can link to it, or know where it is on your computer
4) Click the 'User CP' link up near the top left
5) Select the 'Edit Avatar' choice from the menu on the left.
6) Either enter the URL of where your tiny image is on the web or where to upload it from your computer and click 'save changes'.
For adding your link at the bottom you choose the 'Edit Signature' choice (the next one down from 'Edit Avatar')