Recently there has been a small update released for Redsnow by of course the iPhone Dev Team has. It which will now report or notify you whenever a selected .png image file is invalid when using custom tethered boot logo option.
For all those using the custom tethered bootlogo cmd line option in redsn0w, rc8 now reports why a given png is invalid
Everything else mostly remains the same as the last release. Follow the the step by step guide here to learn on how to quickly boot your tethered jailbroken iPhone, iPad or iPod touch on iOS 4.2.1 using Redsn0w.