Well, my application does not need to return anything to poser, but merely to show a thumbnail if there is a poser RSR and the PNG is absent.
Of course when browsing through the library, poser creates the png files, but in the case of Advanced Library, if there is no PNG thumbnail yet, no image is shown.
The SenoSoft's P3dO paid version utility  apparently can batch create the needed thumbnails, but I intend to make a utility that does not force my customer to purchase another software package.

The workflow I found useful is:

a) Convert the poser RSR to mac PICT format.  I found a lot of information about the fact that poser RSR are in fact mac resources containing a PICT image. The switch header method is wonderful. So far, I still have problems with transparency, but will try Dizzi's suggestion.

b) Convert the PICT to PNG with an appropriate graphics converter library. I use delphi, and there are a number of graphics manipulation and conversion libraries that can be embedded into the application.

JD