I don't think there is one answer here but you may find these suggestions helpful:-- If you save the rendered image as a TIF you can then use its Alpha channel as a mask.

• If you want to render shadows in Poser you will have to put something for the shadow to fall on. Either the ground plane or maybe a flattened cylinder from the Props library.
• If you colour the background in a shade not used in the figure, (bright green for example), it will make selecting it easier in post work.