pnmdepth: 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/pnmdepth - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmdepth - from package: netpbm

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

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