I noticed the images in the Modx manager had no thumbnails after getting Modx working on IIS 6.0 and Windows Server 2003. While the installation script for Modx is excellent, I did not get a warning about the missing GD library. How would one know that the reason the thumbnails are missing is due to the GD library? I can only say I know because I know. If you have a PHP application with missing thumbnails, it is likely GD is not enabled.
How to check if you have GD. Simply make a PHP info page and look for a section named gd. If you do not see one in your info page, open your php.ini file (likely in your Program Files\PHP\ folder) and look for ;extension=php_gd2.dll. Remove the semicolon and save the file. Check your EXT folder to be sure you have the dll located there. Make sure you only have one copy of the dll and in only one location. The contents of your info page should tell you where the extension should live.
After saving the php.ini reload your php info page. Still don’t see it? Navigate to your IIS Manager and refresh your default application pool. Open IIS Manager->Server->Application Pools->DefaultAppPool – Right Click->Refresh. Now reload your info page and you should see gd included in the output.