Once you have the pictures taken, you'll need to upload them to an image hosting site. I'm most familiar with photobucket.com Once the picture is uploaded, you'll need to reduce the image size so that it is no bigger than 80 x 80. After all of that is done, copy and paste the URL that is below the picture into your profile on here. There is a spot for it at the bottom.
My old one was bigger than that, lol. Photobucket changed some things around and when you resize, they have a resize for avatar button. However, it's not 80x80.