rgb2ycbcr: 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/rgb2ycbcr - from package: libtiff

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

On Debian possible default full paths are:

/usr/bin/rgb2ycbcr - from package: libtiff-tools

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

Error in other languages:
rgb2ycbcr: bevel nie gevind nie
rgb2ycbcr: comandă negăsită
rgb2ycbcr: comando não encontrado
rgb2ycbcr: commande introuvable
rgb2ycbcr: command not found
rgb2ycbcr: käsku ei ole
rgb2ycbcr: không tìm thấy lệnh
rgb2ycbcr: komanda nerasta
rgb2ycbcr: Kommando nicht gefunden.
rgb2ycbcr: kommandot finns inte
rgb2ycbcr: komut yok
rgb2ycbcr: nie znaleziono polecenia
rgb2ycbcr: níor aimsíodh an t-ordú
rgb2ycbcr: no se encontró la orden
rgb2ycbcr: no s'ha trobat l'ordre
rgb2ycbcr: opdracht niet gevonden
rgb2ycbcr: parancs nem található
rgb2ycbcr: perintah tidak ditemukan
rgb2ycbcr: príkaz nenájdený
rgb2ycbcr: příkaz nenalezen
rgb2ycbcr: команда не найдена
rgb2ycbcr: командата не е открита
rgb2ycbcr: 命令找不到
rgb2ycbcr: コマンドが見つかりません