yuvtoppm: 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/yuvtoppm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/yuvtoppm - from package: netpbm

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

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