ppmtopi1: 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/ppmtopi1 - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtopi1 - from package: netpbm

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

Error in other languages:
ppmtopi1: bevel nie gevind nie
ppmtopi1: comandă negăsită
ppmtopi1: comando não encontrado
ppmtopi1: commande introuvable
ppmtopi1: command not found
ppmtopi1: käsku ei ole
ppmtopi1: không tìm thấy lệnh
ppmtopi1: komanda nerasta
ppmtopi1: Kommando nicht gefunden.
ppmtopi1: kommandot finns inte
ppmtopi1: komut yok
ppmtopi1: nie znaleziono polecenia
ppmtopi1: níor aimsíodh an t-ordú
ppmtopi1: no se encontró la orden
ppmtopi1: no s'ha trobat l'ordre
ppmtopi1: opdracht niet gevonden
ppmtopi1: parancs nem található
ppmtopi1: perintah tidak ditemukan
ppmtopi1: príkaz nenájdený
ppmtopi1: příkaz nenalezen
ppmtopi1: команда не найдена
ppmtopi1: командата не е открита
ppmtopi1: 命令找不到
ppmtopi1: コマンドが見つかりません