pnmquant: 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/pnmquant - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmquant - from package: netpbm

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

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