I came across this PHP error after installing the Wordpress plugin Contact Form 7 and then using Really Simple CAPTCHA. The exact error given was:
Fatal error: Call to undefined function imagetypes() in ... on line 201.
After searching around for a while, I found that the GD library is required on the server, and (on linux at least) it's a simple case of installing the library from the repository. So on Ubuntu/Debian, run:
sudo apt-get install php5-gd
and on Redhat/CentOS, etc.
sudo yum install php-gd
should do the trick. This will install several dependencies, most importantly - the underlying C Library.
Obviously if you don't have full control over your server, you will have to ask your hosting company or administrator to do this for you. (argh!)