You need to add an alpha channel to the PNG. I don't know why they even open in the first place, but replacing the files does not work.

If you have Paint Shop Pro it is simple, Photoshop should also be able to do it but I am not as familiar.

1.Open the thumb in PSP
2.In the layers tab, promote to layer
3.click your eraser tool, edit the size to smallest
4.put your cursor over an inconspicuous part(I choose a corner of the thumb) and erase one pixal

As long as you promoted the thumb to a layer, you have just added alpha data to your PNG.

5.click file, save

If you do this while Poser is open, you will have to reopen the library to see your changes.
The whole process takes about 20 seconds once you get the hang of it. I have never had a reoccurance.

Mark