webpng: 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/webpng - from package: gd-progs

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

On Debian possible default full paths are:

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

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

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