pnmtosir: 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/pnmtosir - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmtosir - from package: netpbm

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

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