pngquant: 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/pngquant - from package: pngquant

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

On Debian possible default full paths are:

/usr/bin/pngquant - from package: pngquant

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

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