mogrify: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/mogrify - from package: ImageMagick

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mogrify - from package: graphicsmagick-imagemagick-compat

You may install the required package with command apt-get install {package}

Error in other languages:
mogrify: bevel nie gevind nie
mogrify: comandă negăsită
mogrify: comando não encontrado
mogrify: commande introuvable
mogrify: command not found
mogrify: käsku ei ole
mogrify: không tìm thấy lệnh
mogrify: komanda nerasta
mogrify: Kommando nicht gefunden.
mogrify: kommandot finns inte
mogrify: komut yok
mogrify: nie znaleziono polecenia
mogrify: níor aimsíodh an t-ordú
mogrify: no se encontró la orden
mogrify: no s'ha trobat l'ordre
mogrify: opdracht niet gevonden
mogrify: parancs nem található
mogrify: perintah tidak ditemukan
mogrify: príkaz nenájdený
mogrify: příkaz nenalezen
mogrify: команда не найдена
mogrify: командата не е открита
mogrify: 命令找不到
mogrify: コマンドが見つかりません