ppmtorgb3: 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/ppmtorgb3 - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtorgb3 - from package: netpbm

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

Error in other languages:
ppmtorgb3: bevel nie gevind nie
ppmtorgb3: comandă negăsită
ppmtorgb3: comando não encontrado
ppmtorgb3: commande introuvable
ppmtorgb3: command not found
ppmtorgb3: käsku ei ole
ppmtorgb3: không tìm thấy lệnh
ppmtorgb3: komanda nerasta
ppmtorgb3: Kommando nicht gefunden.
ppmtorgb3: kommandot finns inte
ppmtorgb3: komut yok
ppmtorgb3: nie znaleziono polecenia
ppmtorgb3: níor aimsíodh an t-ordú
ppmtorgb3: no se encontró la orden
ppmtorgb3: no s'ha trobat l'ordre
ppmtorgb3: opdracht niet gevonden
ppmtorgb3: parancs nem található
ppmtorgb3: perintah tidak ditemukan
ppmtorgb3: príkaz nenájdený
ppmtorgb3: příkaz nenalezen
ppmtorgb3: команда не найдена
ppmtorgb3: командата не е открита
ppmtorgb3: 命令找不到
ppmtorgb3: コマンドが見つかりません