According to the manual, in Poser 6 it is possible to set an image-map node to "alpha", which means any alpha channel in the image controls whether or not the background input shows, instead of whatever the texturemap is. An obvious use would be to overlay a logo or lettering on an existing texture. The end result should be the same as using a blender math-node, with a control-map taking the place of the alpha channel. Maybe Poser expects something very specific about the alpha channel. I use an old version of Paintshop Pro, which handles the alpha channels that Poser generates without problems. And I've a vague recollection of some trickiness over just what sort of PNG file to use--colour depth or something. But I'm beginning to wonder if this is another of those things in the manual which has only a tenuous connection to reality.