ppmtoneo: 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/ppmtoneo - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtoneo - from package: netpbm

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

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