thumbnail: 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/thumbnail - from package: libtiff

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

On Debian possible default full paths are:

/usr/bin/thumbnail - from package: libtiff-tools

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

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