To explain the why behind what they said. Poser 4 and all prior versions use .rsr files for the library thumbnails (directly derived from the MacOS Poser version), and .bum files for bump files.

ProPack and later versions use .png and .jpg respectively.

Advantages? .png and .jpg are industry standards and thus viewable - and editable - in numerous graphic utilities. And with bump files .jpgs are minuscule compared with .bum files - more correctly .bum files are obscenely large, meaning they are often packaged as .jpg requiring an extra manual step on your part to convert to .bum files for P4. All this is avoided entirely using ProPack or later.

Lastly, there is evidence that MetaCreations incorrectly ported .bum file format when converted from MacOS to Windows, so that's another reason to go to .jpg bump files on the PC platform.

So if you only run p5 (or later) there is no reason or advantage to using the P4 versions. You can also delete all .rsr files in the libraries/ folders too - but only after you first check that a corresponding .png exists. (Poser ProPack or later will create a .png file from a .rsr if one does not exist, at the time that you first open the item within the library - but if you want to do wholesale conversions check out P3d0 utility from Sensoft.)