This shortcode is a quite raw,simple but useful one to me. Allowing you to display a specific user's avatar by setting the user id in the shortcode [avatar id="xxx"].

Usage:

1. Paste the code without "" to the theme's function.php;

2. Add shortcode in your wordpress editor in the form like this: [avatar id="#replace the user_id here(e.g:121)#"]

I could have add more attributes to the shortcode, but I want to keep it simple, so if there is more attr that you want, you can add them by edit the php code.