ppmbrighten: 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/ppmbrighten - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmbrighten - from package: netpbm

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

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