Near the top of the site you will see a 'profile' button. If you go there, you should have another tab of preferences and one will let you change your 'avatar' which are the pictures that we have.
Yes, when you are in your profile, go to "edit profile" : at the bottom of this page you'll see "change avatar"..
Ah, you have to reduce the image you want to 100x100 before, with photoshop i think