pngchunks: 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 Debian possible default full paths are:

/usr/bin/pngchunks - from package: pngtools

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

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