pngcomp: 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/pngcomp - from package: pngnq

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

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