batch_image_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 Debian possible default full paths are:

/usr/bin/batch_image_convert - from package: libvips-tools

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

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