First, you should know that a image like that is very easy to read by almost any OCR. The first one is much more hard to crack.
Anyway, if you don’t want to hack the existing plugin, you can use Really Simple Captcha, it looks exactly like that.
In your form:
<?php
$captcha_instance = new ReallySimpleCaptcha();
$prefix = mt_rand();
$word = $captcha_instance->generate_random_word();
$image = $captcha_instance->generate_image($prefix, $word);
?>
<input type="text" name="captcha" id="captcha" value="" />
<input type="hidden" name="captcha-id" value="<?php echo $prefix; ?>" />
<img src="https://wordpress.stackexchange.com/questions/20665/<?php echo $image; ?>" width="72" height="24" />
after your form is submitted:
$captcha_instance = new ReallySimpleCaptcha();
if(!$captcha_instance->check($_POST['captcha_id'], $_POST['captcha'])) die('wrong captcha!');
// captcha ok, remove the image
$captcha_instance->remove($captcha_id);