imgsize: 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/imgsize - from package: perl-Image-Size

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

On Debian possible default full paths are:

/usr/bin/imgsize - from package: libimage-size-perl

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

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