Isn't the sub-folder just another container for more pictures??? You could use a picture from within the sub-folder as a Screen saver by copying it into C:\Windows (with all the other BMP files) then highlighting it in the Screen Saver dialogue box as usual. Or have I mis-understood the question??
Your right about my memory and my ability to understand the question as well :o(
Been thinking about it all afternoon, then I remembered how I'd done it, only to return to your thread and find that you already knew how to produce a slide-show, which is of course what I had done, and not the answer to your question.
But graham√ [Frank] has come up with the correct way. ;o)