pnmconvol: 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/pnmconvol - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmconvol - from package: netpbm

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

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