I had to go searching... it's been a while since I've updated mine...
Up in the top of your browser where you login, there should be some options for "my profile" and "settings"...
You want "settings"
There are two pictures you can set - your "profile picture" which is what people will see when they go to view your profile, and your "avatar" picture, which is what is shown next to each of your posts. You should be able to see where to edit those on a menu on the left in the 'settings' window.