pngcrush: 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/pngcrush - from package: pngcrush

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

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