ppmtobmp: 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/ppmtobmp - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtobmp - from package: netpbm

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

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