First Step: is to make sure you have the option available:
Go to http://cbox.ws/index.php sign in and next go to Options > Posting Options
Under Message Options check the box next to Allow Avatars.
Next Step: To post an avatar image with your messages, first upload your chosen image to the web.
You can use an image hosting service or your own host. (Photobucket is another available option). Then paste the URL of the image into the “email / url” box, and post your message.
Any questions or comments ask below!