I had this problem last week when preparing my Full & Feathered textures for the MP. My solution: render out all of my previews and save as tifs. Open a new poser file, add a flat square. Apply the preview.tif to the square. Save as a prop. Go into the library folders in Explorer and steal the RSR from the preview square prop and put it in the folder with your model. Now match up the names, so the object and its RSR have the same name. There ...all done. Annoying but it works.