Many things in your profile can be changed to your preference including an Avatar picture and Profile Picture

Click on 'Settings' at the top right. Then on the menu down the left hand side is 'Edit Profile Picture' you can upload an image, cannot be greater than 120x120 pixels though, so you may need to resize it in a paint package, also Avatar's too, pretty much the same as a profile pic but appears in different places, you can upload an image under that in the same way.