pnmscale: 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/pnmscale - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmscale - from package: netpbm

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

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