PHP5-gd

Put this in the httpdocs or public_html directory to see if GD Library is installed.

<?php
 $array = gd_info();
 foreach($array as $key => $value){
 if ($value == 1){
 $value = "yes";
 } elseif ($value == ""){
 $value = "no";
 }
 echo "<b><i>$key:</i></b> $value\n<br />\n";
 }
?>

For Ubuntu you can normally just do apt-get install php5-gd via SSH.

To install it on CentOS 6 with WHM/cPanel you can enable it via WHM > Software > EasyApache - PHP Encryption and Image Manipulation

Alternatively log into your server via SSH. Then you will want to do:

yum install php-gd.x86_64 gd.x86_64 gd-devel.x86_64