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);