ppmtv: 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/ppmtv - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtv - from package: netpbm

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

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