Including both the .rsr and the .png is no problem at all. Many merchants do it already. The users can then select which ones to install depending on Poser version. If they choose to install everything, it's no problem, but it takes up more disk space. When only the .rsr is included, Poser 5 will automatically create a .png the first time that library is opened in Poser. After that, the .rsr can be deleted. I do a ".rsr cleaning session" every now and then after having installed lots of stuff with just .rsr thumbnails...