Unable to parse IPSW

I tried to upgrade my iPhone 3GS from iOS 5.1.1 to 6.1.2 via this tutorial: http://www.iclarified.com/25144/how-to-upgrade-your-iphone-4-3gs-without-updating-the-baseband-windows But in redsn0w when I select iOS 6.1.2 as IPSW, it gives an error "Unable to parse IPSW. Are you sure it's really an IPSW?" http://img593.imageshack.us/img593/1044/redbr.png