Live demo here:
First let's start with the markup we're going to need. Simple is key here, we're going for lightweight HTML - you can customize it with little trouble.
<img src="https://secure.gravatar.com/avatar/0d47338e59943a1baa5d83f81cf6913b?s=140&d=https://a248.e.akamai.net/assets.github.com%2Fimages%2Fgravatars%2Fgravatar-140.png" alt="Sergio Tapia" />
<p>Best comment ever!</p>
Next let's focus on the CSS. I'm using LESS because it's much easier to write styles using nested statements.
border:1px solid #cacaca;
border:5px solid orange;
box-shadow:0px 2px 3px 0px #000000;
The main CSS you should focus on is the styles for the img tag.
Using a border-radius style of 80px gives you a nice round shape, and it looks great pretty much everywhere. Extremely simple and fast!
Now I've taken a few assumptions, for example I assume the picture is going to be square. I would recommend setting a standard height and width for the img tag.
In fact: Coderwall itself does this as well. Check if out, right click in my user icon up top and inspect the CSS applied to it.