how to add captcha to my form

<input type="text" name="formcode" class="inputBox" style="width:150px" size="20">
              <a href="[+action+]"><img align="top" src="manager/includes/veriword.php?rand=<?php echo rand(); ?>" width="148" height="60" alt="If you have trouble reading the code, click on the code itself to generate a new random code." style="border: 1px solid #003399"></a>

    if($_SESSION['veriword']!=$formcode) {
        $output = webLoginAlert("Incorrect form code. Please enter the correct code displayed by the image.").$tpl;
        return;
    }