function Ajax_captcha(){
	var code = $("#code").val();	 // get the code entered 
	if(code.length>1){
	$('#Loading').show(); // show the loader image
	$.post("<?php echo base_url()?>Site/Ajax_Captcha",{
		code: $('#code').val()
	}, function(response){
		$('#Ifno').fadeOut();
		setTimeout("RemoveAJAXCaptcha('Info', '"+escape(response)+"')", 400);
	});
	}
	return false;
}
// this function will hide the loader image and show the result as inner html
function RemoveAJAXCaptcha(id, response){
  $('#Loading').hide();
  $('#'+id).html(unescape(response));
  $('#'+id).fadeIn();
}

<input id="code" name="code" type="text" value="" onblur="return Ajax_captcha();" />
<span id="Info"></span>
<span id="Loading">
<img src="<?php echo base_url()?>images/loader.gif" alt="" />
</span>

function Ajax_Captcha($code,$this->session->userdata('yourSessionVarOfCaptcha'))
{
  if ($code==strtoupper($this->session->userdata('yourSessionVarOfCaptcha')))
  {
       return true;
  }
  else
  {
      return false;
  }
}