convert: 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/convert - from package: ImageMagick

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

On Debian possible default full paths are:

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

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

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