image_server: 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 Debian possible default full paths are:

/usr/bin/image_server - from package: ctn

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

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