Everything points to the fact that pmx to png conversion is related to the MikuMikuDance software, to be more specific, export of some parts or preview views of .pmx models as plain PNG pictures. We have not found any specifics about this, just that MikuMikuDance supports export to PNG format.
But it might also be related to attempts to recover graphics from old Adobe PagerMaker documents (also .pmx files), in which case you have to obtain copy of PageMaker.