Open the offending PNGs in Photoshop - make sure you have the layer window open and check if the  PNG displays as background or Layer0. If it displays as background then this is the reason why it won't show up in Poser.

To fix this problem: go to Layer - new layer from background and resave the PNG. It should now show up in Poser.

Every organisation rests upon a mountain of secrets ~ Julian Assange