In Mac OS, prior to the use of pngs, I believe that Martin C.'s MacConverter stripped out the data fork (everything but the first and last line) from the rsr file and added it to the PICT resource of the cr2 file. But now that they use pngs, there isn't any reason to deal with rsr thumbnail files, at least on the Mac side. With the exception of Poser 3 and 4, I guess, if that's the idea here.