There is a donationware piece of software for the Mac that does the RSR to PNG conversion quite nicely.  I used it on my 39.53 GB Runtime folder and it took care of everything in a few minutes.

The app is called RSRConv and can be found here:
http://braintrigger.com/software/RSRConv/

The only issue I had was the trademark symbol in the Poser folder name containing my primary runtime.  Once I renamed the folder, the thumbnails appeared in the library.