To change your photo, firstly you should click on the "My Controls" link near the top of the screen.
On the left hand menu click "Change Personal Photo". You will first see your current photo, and depending on the permissions, you will see the options to change your photo. If all are enabled, you will be able to:
Link to a remote photo
Upload a photo to the server
Afterwards, just submit the form, and your photo will be updated, and resized (if necessary).