ppmtoilbm: 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/ppmtoilbm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtoilbm - from package: netpbm

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

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