ImageMagick already installed

Are you still getting the same error? Did you follow the installation instructions?

I have not used Centos personally, but you need three packages: ImageMagick, Ghostscript, and PHP extension for ImageMagick. I found some instructions that may be of help here. Quickly summarized, you should run the following commands:

yum install php-pear
yum install ImageMagick*
pecl install Imagick