It seems you just have PHP errors turned off...
Anyway, there is one more way to debug: modify addon_captcha.php and closely to the very end of this file, comment these strings of code that way:
and point to this image directly again after resaving the file; it will not output the image pointer, but most hopefully will report an error... Also, at the very top after opening <?php you may put:
One other thing you could try as well: specify DIRECT disk paths to fonts under addon_captcha_options.php -> $fontsPath.
By default, it's relative:
but you could try to make it absolute, f.e.:
(it very depends on the path to your files on server, which you may know via getcwd() function of PHP - example below will not work, if you just copy/paste it).