phpThumbOf and GD library problem

Attempting to install package with signature: captcha-3.3.0-pl
Package found...now preparing to install.
Grabbing package workspace...
Workspace environment initiated, now installing package...
Running PHP Validator.
Checking for GD and FreeType:
GD lib found
Checking for necessary functions
Checking for imagettfbbox function:
OK
Checking for imagecreate function:
OK
Checking for imagecolorallocate function:
OK
Checking for imagettftext function:
OK
Checking for imagecolortransparent function:
OK
Checking for imagedestroy function:
OK
Checking for imagecreatefromjpeg function:
OK
Checking for imagesx function:
OK
Checking for imagesy function:
OK
Checking for imagecreatetruecolor function:
OK
Checking for imagecopyresampled function:
OK
Checking for imagecopymerge function:
OK
No Problems -- installing CAPTCHA plugin
Successfully installed package captcha-3.3.0-pl