Once you're logged in, click "Edit Profile" there at the very top of the the site. There's an option there under "Account Settings" called "Upload Photo."
Click on this link and use the form on the next page to upload your photo.
Photos must by 100x100 pixels or smaller at 72dpi, and must be under 50kb.
You can use any photo editing program (even the Paint program built into Windows) to resize your image.