pamtohdiff: 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/pamtohdiff - from package: netpbm-progs

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

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