mem_image: 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/mem_image - from package: LibRaw-samples

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

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