rgb3toppm: 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/rgb3toppm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/rgb3toppm - from package: netpbm

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

Error in other languages:
rgb3toppm: bevel nie gevind nie
rgb3toppm: comandă negăsită
rgb3toppm: comando não encontrado
rgb3toppm: commande introuvable
rgb3toppm: command not found
rgb3toppm: käsku ei ole
rgb3toppm: không tìm thấy lệnh
rgb3toppm: komanda nerasta
rgb3toppm: Kommando nicht gefunden.
rgb3toppm: kommandot finns inte
rgb3toppm: komut yok
rgb3toppm: nie znaleziono polecenia
rgb3toppm: níor aimsíodh an t-ordú
rgb3toppm: no se encontró la orden
rgb3toppm: no s'ha trobat l'ordre
rgb3toppm: opdracht niet gevonden
rgb3toppm: parancs nem található
rgb3toppm: perintah tidak ditemukan
rgb3toppm: príkaz nenájdený
rgb3toppm: příkaz nenalezen
rgb3toppm: команда не найдена
rgb3toppm: командата не е открита
rgb3toppm: 命令找不到
rgb3toppm: コマンドが見つかりません