diffimg: 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/diffimg - from package: graphviz

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

On Debian possible default full paths are:

/usr/bin/diffimg - from package: graphviz

You may install the required package with command apt-get install {package}

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