ppmrainbow: 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/ppmrainbow - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmrainbow - from package: netpbm

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

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