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 the gd_test.php file to your server and opening it in your browser. Unless you get a red image saying WORKS you will need to contact your host and ask them to enable GD library for PHP.

If you DO get the WORKS message the problem are probably sessions. If you made any modifications to the GBook files try using the original files (modify just the settings.php file).

Test sessions:

1. Download Session test files, upload them to your server and open session_test.php in your browser.
    
2. Click the CLICK HERE FOR PAGE 2 link.
    

If you get "$_SESSION['test'] is set to: FALSE" contact your hosting company and ask them to check why PHP sessions aren't working normally. If you get TRUE instead of FALSE the problem is somewhere else.