pnginfo: 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/pnginfo - from package: pngtools

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

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