I do alot of Compositing, with Photoshop.  Like when I ca'nt sdecide wich way I'd like to render something; I just render both ways, add each render to a Photoshop Layer, and then play with the Layer's Opacity.  Or when I just ca'nt get two different Objects to render the way I like them, within the same scene, at the same time.  Then I just render each seperate, and put them together with Photoshop!

Photoshop also has some really great tools, for brushing up your images.  I use the Clone Tool, and the Blur tools alot, to get rid of unwanted shadows, like joint bend creases, etc.

Acadia - .PSD is good, but it still a Compressed format.  Window's native .BMP is the best(most raw) format, to archive your images.   I would think it would be best to save, from Poser, as .BMP; and then let  the newest version of Photoshop create your  .PSDs for you.