gdparttopng: 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/gdparttopng - from package: gd-progs

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

On Debian possible default full paths are:

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

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