ppmqvga: 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/ppmqvga - from package: netpbm

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

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