mtvtoppm: 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/mtvtoppm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/mtvtoppm - from package: netpbm

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

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