Quote - "Why not just render to JPG instead of BMP?"

Because then you lose quality at the source, due to compression. If you save to BMP, and you convert to JPG to fit to whatever file size restrictions some site has, you can always go back to your BMP and convert to different file sizes. If you save directly to JPG, the quality is lost at the source and there's no way to get it back.

Again, I don't think that this loss of quality noticeable in Renderosity galleries has nothing to do with the way an image is converted in Vue or PS or whatever. The exact same image posted in Renderosity or in Cornucopia3D looks completely different. In C3D it looks much sharper.