GD Graphics Library2 (GD2)
GD2 (version 2 of the graphics library) is a graphics library that is installed on almost 100% of all web servers and will for instance make it possible to automatically generate thumbnails for images or VVC images, however sometimes it can stop working.

If you have a single script, or multiple scripts that cannot generate visual verification codes or image thumbnails then you should look at reinstalling your GD image support. Your server admin or web host can help you with this.

GD is something your server administrator/host has to reinstall/reload/configure etc but you can test the functions using a GD test script such as the one listed here: (This is what I use)
http://www.dagondesign.com/articles/checking-for-gd-support-on-your-server/