The anti-SPAM security image is blank.
For the image to work your server must have GD library installed.
You can test if your server supports all the required functions by uploading gd_test.php file to your server and open it in your browser.
If you get an error or empty page when opening gd_test.php you should either ask your hosting company to install the GD library for PHP or disable the anti-SPAM image in the HESK admin panel under "Settings".