First you must join and log in. After you have logged in, this process is simple.

Go to your profile by clicking on the "My Profile" link on the user menu on the left side.

After you are in your profile area, you will see an "Edit" tab (This tabe may take a few seconds to load),  if you put your mouse cursor over it, you will see a drop down menu with a couple of options. Choose the one that says "Update Your Image"

That's it! Pick out a great picture by browsing your computer and then click upload! Just make sure it's no larger then 3MB.