ppmwheel: 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/ppmwheel - from package: netpbm-progs

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

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