Security Image Not Displaying

[dooper3]

I am trying to get the security image to display within a form on my website so that users can then type in what they see. The image displays fine when on it's own, but when I try and write it out in the form it doesn't render and just generates a load of wierd code (not php). Does anyone know why this is? The image is generated by a file called securityimage.php, which I include at the top of the page I want a security image in as it has it's own headers (to tell the computer not to cache it), then in the form I use the include security_image_display.php which just has the imagepng() and imagedestroy() functions in it.
I also tried getting rid of the custom headers and just having the securityimage.php and security_image_display.php files as one include but alas the same problem occurs! Help!

Thanks,
Charlie

P.S. The code for securityimage.php is largely based on this code here - http://www.codedfx.com/index.php?id=tutorialfull&did=42 - but modified slightly in terms of the colours it uses, font and text positioning.