"I thought of doing that many times but isn't it a problem for shadows ? How will shadow of the latest characters behave on the background ? I suppose it's OK for distant scenes but may be not for close characters?"

Import the image as a background, not a texture applied to a panel.

Poser doesn't rerender the backgroud so there's very little overhead.

the shadows don't show on the background, just don't keep moving the lights around or you will get different lighting on different figures.

You will have to try and not have the fact that shadows don't show on the background be a problem.  It could look strange if one character has a cast shadow on it and the one next to it doesn't.

Mike

If you shoot a mime, do you need a silencer?