xt-customize-image: 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/xt-customize-image - from package: xen-tools

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

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