If none of the examples above to work there is a couple of things that needs to be checked. If not any of these quick fixes solves the problem then it is best to read the long version of the installation instructions before continuing the trouble shooting.
Verify that the GD library is really working by running
                                phpinfo() and check that your output includes a GD
                            section which should show similar information as the image below
                        
Make sure you have enabled strict error checking and disabled output
                            buffering in php.ini. This might not solve the
                            problem by itself but it will enable (better) error messages to trouble
                            shoot the installation.
Make sure that you are using a recent version of PHP (e.g. ≥ PHP 5.2.x)
Make sure that the path to the TTF directories are correctly specified
                            in jpg-cnfig.inc.php