ppmrough: 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/ppmrough - from package: netpbm-progs

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

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