First, log into your account.
On the Control Panel, go to Quick Links, and then click on "Edit Profile."
That will bring you to a new screen. At the top of that screen is a tab labelled "Profile Picture." Click on it to arrive at another screen.
From here, click on "Browse" to go through the files you've saved onto your PC. Select whatever photo you want to have as your avatar, click ok, then click on "Upload."
After a few seconds, you'll see the avatar you selected on the screen. If that's the one you want to go with, click "save changes" and you're set. Note that nothing will appear to have changed: that's fine.
If you ever want to change your avatar, return to this screen, click on "Remove Picture" (which is located directly below the avatar), and then repeat the previous steps, beginning with clicking on "Browse." As a courtesy to other users, please try not to change your avatar too much, as most of us identify people by the image first, with the name being checked only afterwards.