imsize: 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 Debian possible default full paths are:

/usr/bin/imsize - from package: wcstools

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

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