iconvert: 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/iconvert - from package: OpenImageIO-utils

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

On Debian possible default full paths are:

/usr/bin/iconvert - from package: openimageio-tools

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

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