advpng: 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/advpng - from package: advancecomp

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

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