ppmtoxpm: 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/ppmtoxpm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtoxpm - from package: netpbm

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

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