You must have GD2 module installed with PHP.
If u get imagecreate() not found, it's means you don't have GD2 installed.
You can install it easly on windows by open php.ini and uncomment extension=php_gd2.dll line. Also check that extension_dir in php.ini is correct.
You must have arial.ttf available for the script to work.
If the picture is shown without numbers (only grid) u need to download
arial.ttf from http://www.webpagepublicity.com/free-fonts-a4.html.
Save arial.ttf in the same directory of the script. Notice it's arial.ttf
and not Arial.ttf.