Use javascript to check http status codes

I want to display profile pictures from gravatar for only those users who have a picture set. Doing this server side means doing around 100 HEAD requests to gravatar for checking 404 codes and appropriately outputting img tags for each request.

So, I want to implement a javascript function where I can just output 100 urls for which javascript can check the http status codes and output the appropriate image tags dynamically. Is that even possible? How?

The keyword you're missing is "status code" (that's what we collectively call all the HTTP response codes of 200, 404, 500, etc). I'm going to assume you're using jQuery, in which case, all the documentation you need for doing AJAX is at http://api.jquery.com/jQuery.ajax/

Here's a simple example for a request that displays an alert, but only if a 404 status code is returned (lifted almost verbatim the link above):

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>
   $(function() {
      var url = "some_url";
      $.ajax(url,
      {
         statusCode: {
         404: function() {
            alert('page not found');
         }
      }
   });   
});
</script>