pngcheck: command not found

Have You tried accessing the binary with the full path?

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/pngcheck - from package: pngcheck

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