It seems to be a funny little bug... Sorry for my first reply about the background picture (deleted now) - that was wrong. Poser keeps a copy of the background picture on Mac, so my first guess was that it started to do the same on PC. However, I now re-checked the PC version and the .rsr files for .pz3 are void - there is no real content in it, just 286 bytes of plain overhead. It looks like that Poser started to create them by accident now, just ignore it.