The latest opdate of Poser 12 features the fix for a real pain: to dissect your .png to get a separate mask file.

The Cycles ImageTexture node now outputs the transparency (alpha) channel correctly. So, you need no mask anymore to crowd your scene with 'poster' figures.
The below image is simply an old Roxie .png I had, as cycles image texture on two squares. Note it correctly casts shadows. I did not make the squares 'look' at the camera, so I could show the principle.

The dead-simple shader I used for the PhysicalSurface node on the left square.

PoserSurface a little more complicated, on square on the right:

No separate masks needed anymore when you dare to go into the cycles nodes lists. Hurray!