There is two kinds of "imagemagick". The one is the imagemagick shell program that needs exec or shellexec permissions from a subscription to be used, the other one the corresponding PHP module.
Test on the OS level whether this is installed:
# convert -version
Will return version information, if installed. Does not depend on OS version. File location is normally /usr/bin/convert. If you cannot find it there, try
# updatedb
# locate convert | less
If not installed, it can be installed like this:
On Debian and Ubuntu for example:
# sudo apt-get install imagemagick
On Redhat and Centos for example (normally installed already):
# yum install ImageMagick
The second component is "imagick", a PHP component. This should be available by default on Plesk servers. Imagick does not need permissions for exec() or shellexec() and is therefor widely used. Some programs like Typo3 however run the convert utility through exec()/shellexec(), which is a problem for secure shared web space accounts. To test whether imagick module is available in PHP, simply call the phpinfo() function, e.g.
<?php phpinfo() ?>
and check the output the for imagick module.