Well, I know a way using jQuery.
<div id="icon"><img id="wicon" src="" alt="Weather icon"></div>
At the HTML above you see the unique thing missing is the src attribute, so let’s fill it with some jQuery and JavaScript.
You may create a variable to hold the icon code provided by the API like:
var iconcode = a.weather[0].icon;
After it you should concatenate this var iconcode with the url that contains the icons, like:
var iconurl = "http://openweathermap.org/img/w/" + iconcode + ".png";
Finally just change src attribute in the DOM by doing this:
$('#wicon').attr('src', iconurl);