I would typically make the assumption that this has been resolved but I noticed that I am not seeing the captcha image in any of the previously posted links, including pages you indicated to be working for you.
There are a couple ways that you could remedy this, but the easiest is probably to simply put a leading slash in front of the reference to your image (php) file. Right now you're using relative pathing so when you go to this url: "http://www.mattaproductions.com/includes/example_form.php", the image doesn't show up because the img tag is looking for it as:
This is telling the page to actually try and find the file at "http://www.mattaproductions.com/includes/includes/securimage_show.php" which is invalid. If you were instead simply to change the tag to: it would look at "http://www.mattaproductions.com/includes/securimage_show.php" which is valid.