First, you have to find a place to host your picture. You can host here by clicking on the "Photos" link at the top of this page. Then you log in with your normal password/login. You then click on Upload Pictures and choose Members Area and upload your pic(s). They will be under your folder. Note the URL underneath the image-- you'll want to copy and paste it (see below).
You'll then be able to use them as your 'avatar' -- the picture under users' names. To do this, click on My Home at the top of this page and click on the first 'Edit' button and enter the URL of the avatar image.