How can I use Shadows in Poser with Background Images? No Cyclorama, just a Picture. In Max I can do this with a Matte Shadow Material on a Plane. Is there a way to do the same in Poser? Maybe with Postwork in PS?

You're probably better off with postwork, although there is a shadow-catcher material for Poser 5. Results may vary depending upon your lighting scheme and the complexity of the shadow-catching objects. http://www.runtimedna.com/messages.ez?Form.ShowMessage=76990 Poser 6 has a built-in shadow-catching feature, along with an option to render shadows separately from the rest of the scene.