ppmforge: 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/ppmforge - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmforge - from package: netpbm

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

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