What are they, how do I create them and are they terribly important?

Well, there are 2 .rsr, one sits in the Geometries folder along with the .object file with coordinates, the other is a thumbnail used in Poser 3 and Poser 4, Poser Pro uses .png and so does Poser 5 for their thumbnails. So, which one are you talking about? Yes, they are important. Sharen

Poser will automatically generate the rsr that sits in the geometries folder. Actually, with many of P4's crashes, you tend to need to delete this and allow it to regenerate anew. You need to nothing to create it but load up the cr2 that calls for that object. They can be deleted without causing any problems. (It will just take longer for poser to load that obj next time until it recreates it.) The thumb rsr's will auto-generate when you add something to your library through poser. Or, there are some nice tutorials around regarding making nicer ones. These are only important if you want to see what you have :D lol

OK, it's for thumbnails, i have the Png, how do i convert them to rsr?

I usually use p3d0 explorer, since I find it essential in helping me keep track of my mess. There is a plug in that either comes with it or you can add to it. (I forget) that will allow you to convert a whole folder of pngs to rsrs. You can find it here in freestuff, or I think the site link is www.senosoft.com. Great piece of software.

There is a third and fourth rsr file, used in P4, PP and P5, which are in the main Poser folder. Don't delete those ones! It may be best to set the Poser.rsr one to read-only and make a back-up of it just in case you ever do a complete rsr cleanse!