pngtogd2: 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 CentOS possible default full paths are:

/usr/bin/pngtogd2 - from package: gd-progs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/pngtogd2 - from package: libgd-tools

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

Error in other languages:
pngtogd2: bevel nie gevind nie
pngtogd2: comandă negăsită
pngtogd2: comando não encontrado
pngtogd2: commande introuvable
pngtogd2: command not found
pngtogd2: käsku ei ole
pngtogd2: không tìm thấy lệnh
pngtogd2: komanda nerasta
pngtogd2: Kommando nicht gefunden.
pngtogd2: kommandot finns inte
pngtogd2: komut yok
pngtogd2: nie znaleziono polecenia
pngtogd2: níor aimsíodh an t-ordú
pngtogd2: no se encontró la orden
pngtogd2: no s'ha trobat l'ordre
pngtogd2: opdracht niet gevonden
pngtogd2: parancs nem található
pngtogd2: perintah tidak ditemukan
pngtogd2: príkaz nenájdený
pngtogd2: příkaz nenalezen
pngtogd2: команда не найдена
pngtogd2: командата не е открита
pngtogd2: 命令找不到
pngtogd2: コマンドが見つかりません