if you know what rooting is, but you are not rooted, go here
forums.androidcentral.com/lg-optimus-s-rooting-roms-hacks/131297-how-rooting-ls670zvd-zvh-100-automated-no-revert-needed.html
if you are completely new to this, go here
forums.androidcentral.com/lg-optimus-s-rooting-roms-hacks/153310-info-welcome-basic-optimus-s-guide-newcomers-info.html
if you don't know whether or not you are rooted, than you are not rooted.
if you know what rooting is and you are sure that you are rooted
type
su
before you enter the flash image command