ppmtoyuv: 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/ppmtoyuv - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtoyuv - from package: netpbm

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

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