ppmtomitsu: 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/ppmtomitsu - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtomitsu - from package: netpbm

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

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