What Are "_R.jpg" Versions Of Thumbnails Supposed To Do?

While moving our store to a new server we noticed 404 entries generated by CS Cart attempting to load nonexistent image files. For example (redacted for security reasons):-


The image without the _r suffix ("/images/thumbnails/600/450/detailed/15/photo-of-something-we-sell.jpg") exists in that directory and can be loaded. Are these "_r" images part of the main CS Cart installation itself, and (if so) what are they supposed to do?

(This doesn't appear to cause a problem with our store, but we noticed it was happening on the old server as well.)

- Smich