imagemath: command not found

Have You tried accessing the binary with the full path?

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/lib/cmtk/bin/imagemath - from package: cmtk

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