display: 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/display - from package: ImageMagick

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

On Debian possible default full paths are:

/usr/bin/display - from package: graphicsmagick-imagemagick-compat

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

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