Poser will only interpret black and white transparency maps, not alpha channels.

If you have a png file where the transparent parts are, well, transparent take the file into an image editor, create a black fill layer underneath, merge down, flatten image, and save. Black is transparent in Poser. White opaque. Half tones degrees of transparency. If it's a color image with an alpha channel merge it with an opaque layer and apply that as you would a color texture. Take a copy of that file, select the outline of the color information and fill that area with white (opaque) and the background as black and use as a trans map. UV map your object, and fit the image to the UV Template. You will have 2 files, the color diffuse and the black and white trans. Apply as you would a regular texture in the material room with an image map node attached to the transparency socket, when you load the transmap change the gamma to custom 1, black and white maps such as bump, spec maps, displacement, and transparency must be custom gamma 1. Apply the color version as you would a diffuse texture map.

Also See this...

https://www.renderosity.com/rr/mod/forumpro/?thread_id=2965163

If you need further images showing how to make the 2 maps let me know, I'll whip something up real quick.