Depending how old your content is and how current your Poser version, you can delete rsr files if png files are present in the same folders.  These files create the icons you see in the Poser Library.  I believe Poser 6 (or possible version 5) switched formats, so you really don't require both formats to be present.  For older files retaining the rsr format alone, D/L a free version of P3DO Explore.  It can easily convert the rsr to png and delete them at the push of a button.  No need for redundant files cluttering your folders.