Or, you can mouse over your user name in the upper right hand side of the screen and chose User Profile from the drop down. In this area you can change the color scheme of your toolbar, change your contact info or password, as well as add a bio to your profile, if you want.
Scroll down the page to the Avatar section, and click on the “Choose File” button to upload your image. The image is cropped to a square by the system, so I would recommend a square image. If you re-size the photo to 125 x 125 (pixels) you will know how you avatar will appear. Scroll to the bottom of the page and click Update Profile add your avatar.