(1) Load the prop into your scene, delete everything else.

(2) Render: Render Dimensions -> Check "Render to exact resolution". Set the size to 91x91.

(3) Render, then choose File: Export: Image. Change the file type to save to PNG. Browse to the folder where the prop's PP2/PNG are, and click on the existing PNG to give the new one the same name, saying yes to overwrite existing file.

Done.

This is easier to do if you do it directly after saving the prop, since Poser will immediately browse to the correct folder.