Problem with Captcha Helper

[eluser]jvk22[/eluser]
I'm trying to get a captcha to display, but its not working and I'm not sure what I'm doing wrong. (I'm using the yahoo url just as a test image, once I figure out how to make this work I will change the image)

Code:

$this->load->helper('captcha');
$vals = array(
'word' => 'Random word',
    'img_path' => './events/',
    'img_url' => 'http://l.yimg.com/a/i/mntl/ww/events/p.gif'
    );

$cap = create_captcha($vals);



echo 'Submit the word you see below:';
echo $cap['image'];
echo '<input type="text" name="captcha" value="" />';