hdifftopam: command not found

Have You tried accessing the binary with the full path?

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/hdifftopam - from package: netpbm-progs

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