If you're doing a large scene with lots of shadows then it's best to do it in 2 passes.

Do your colour image first with shadows "unchecked" then save as a png or tiff [I prefer png]

Then render again but this time "check" your shadoes and also check "shadows only"

This will render only the shadows, nothing else so all you'll see is a white image with black where the shadows are. save as a png or tiff.

Take both images into photoshop or whatever you use and put them together as layers.

Select your shadow layer and make it "multiply", this will give you your full image as if you'd done it in poser complete, flatten your layers and save as a jpg, job done!

 

Injustice will be avenged.
Cofiwch Dryweryn.