Extensis has a plug in to make masks, but its cheaper to jsut use the alpha channel that poser generates. I export image as PSD ( photoshop format) go to the alpha channels, click the rgb channel, go to bottom of pallette, click make selection go back to layers- make sure layer one is selected ( that should be your subject) and ctrl c it into my new document. You can also use the select-> modify->contract selection to bring in your selection by one pixel, ensuring you get no fringes or edges. Check the tolerence on your tools,.. you may want to set them for zero.