Open your Web browser and visit the official Steam website (see References). Click "Login" in the upper-right corner. Type your Steam user name and password into the boxes and click "Sign In."
Select the "Settings" option on your main account page. Select "Edit Profile" from the page that appears.
Click the "Browse" button beneath your existing Steam account picture. Select the new image you want to use with the window on screen. With the new image selected, click "OK" to return to your main Steam account page.
Click "Save." This will change your account picture on the Steam online service.