OK, first you need to have the picture you want to use on you computer, it helps if it is a picture of a cartoon duck.

Next go to your profile page, just click on your user name on the upper right corner of any page that you are on.

Then in the blue bar near the top of the page on the far left is "My Profile" click that and along the left edge of the page will be links to were you can add a profile picture, and an icon picture and basically edit your profile to your liking.