Go to your profile.

Scroll down to "Edit my profile".

Click "Edit" beside "Personal Information".

Scroll down the page again, you will come to a section about uploading an avatar. You can upload it if it doesn't exceed 80x81pixels, and you can either upload it from your computer, or put it on a site such as PhotoBucket, and use the HTML link they provide with the photo to use instead.

Does that help any?