I believe Poser, just like Studio, converts the textures to mip maps that use a lossless format similar to TIFF.  If that is the case the actual resolution of the image will determine the memory used in, not the image's file size.  Because of the mip mapping conversion I often reduce large res images.  In order to get good result the images need to be high quality to begin with.  Never go below 85%(and 90% is preferred) quality with jpgs, the quality drops off faster than the file size.