pngsplit: 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/pngsplit - from package: pngcheck

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

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