Also if you want to take the kind of screenshots that would benefit from a green screen, Source Filmmaker is a much better bet. You can render a "poster" at 4X resolution with no AA, make a perfect cut-out of whatever part of the scene you wanted using the magic wand tool, and then shrink it down to a quarter of the size with bilinear filtering. That's what I do. There's even a greenscreen model already out there in the SFM workshop.