pngtogd: 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 CentOS possible default full paths are:

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

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

On Debian possible default full paths are:

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

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