ppmtoppm: 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/ppmtoppm - from package: netpbm-progs

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

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