Oh, now I understand why you need an .RSR, you do'nt know what one is.  Sorry, I missed the "what is and..." part of the title to this thread.  Actually, I think everyone did ;  .

A .RSR file is the file that Poser uses to make the thumbnails in all of the Poser Libraries i.e. Figure, Expression, Light, etc.  That's all they are, just a picture.  And whatever, library preset, file(.pz2 for Poses, .MC5 for materials...) has the same name as the .RSR is the file that get run when you double click the .RSR from a Poser Library!

Now what I said up top should be easier to understand.

BTW - RSR Converter is a free utility.  I should have mentioned that also.

Oh, and a good reason for using .PNG instead of .RSR is that Windows Explorer can make thumbnails of .PNGs, but not .RSRs.

That's not to say the Poser can't still read/use .RSRs, it's just they are'nt favored really.