ppmflash: 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/ppmflash - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmflash - from package: netpbm

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

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