pnmpsnr: 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/pnmpsnr - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmpsnr - from package: netpbm

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

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