once the file has been saved as a PPS document then it is no longer a collection of individual files, more of a complete set all rolled into one.

If you are using OS X 10.2 or above, you should be able to use the utility called GRAB which is in the utilities folder. Using this, you can take a screen shot of just the picture and save it as whatever you want.
Practice first, it is really quite easy.
Renaming files from .??? to .JPG will not convert that file into that format. To convert pictures, you will need something like Graphic Convertor, available on-line as a try before you buy item. Just Google for it

Hope this helps

P