rasttopnm: command not found

Have You tried accessing the binary with the full path?

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/rasttopnm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/rasttopnm - from package: netpbm

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