ppmchange: 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/ppmchange - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmchange - from package: netpbm

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

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