The difference in lighting could be because of different gamma settings between Poser and Photoshop. I found that images taken from Poser into Photoshop have a slightly reddish tinge to them, and I corrected that by creating a .ahu file that automatically adjusts the hue and saturation. Also, try using the "save as" function for your render instead of "export as". For some strange reason, it seems to make a difference.