Liberty three has screenshot in the power off options menu when you hold the power button so that it could be used during any phone function, except for boot screens. I don't know why it wasn't used in eclipse. I'll add it to my discovered bugs/problems and suggestions post on the eclipse rom thread when I find all of them.
My phone is not rooted and the above link is what works for my phone. I spent the money as it was worth it. Install it and it works. There is a trial version rhett let's you take a few pictures for free. The dev has detail notes on settings and what you may need to do to get color correct.