imgmerge: 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/imgmerge - from package: xpaint

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

On Debian possible default full paths are:

/usr/bin/imgmerge - from package: xpaint

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

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