RSR was a proprietary Poser format, I think.

Though according to the documentation I've found, Poser 4 was the last time it was required. ProPack, Poser 5, Poser 6, and Poser 7 were supposed to be able to read both. After that, it was PNG only.

Royfitz, are you sure you're running Poser 5 and not Poser 4? In any case, if you're not upgrading, it might be worth investing in P3DO Explorer, since many other products you are likely to buy or download will have the same issue.