This is just personal preference, but i wouldn't use the gmod construct stage to do your posing in. It's not a very good looking stage, and i think using it in a comic looks quite unproffesional, so i recommend one of the HL2 (if set in that universe) or CSS (handy for any setting really) maps, or creating one yourself that suits your purposes.
Also, i recommend taking your screenshots at high-resoultions (i used 1152x864 for Get The Message) and shrinking 'em down in photoshop or whatever program you are using. They end up looking alot smoother and nicer if shrunk down say 75% or 50%.