Assuming you have Poser Artist (a.k.a., Poser 4), select the ground plane, then open the material panel (CTRL-U) and load each JPEG as a bump map. Poser will convert them to BUM format and save them to the same folders as the originals. If you have Pro Pack or P5, Poser won't convert the files. It can use the JPEGs as-is. You might have to edit your character files so Poser will reference the existing JPEGs instead of BUMs.