Something to consider re using this sub-folder method is that presumably you're going to have a common navbar. Unfortunately using Chris's example above:

Main site - mysite.com/index.htm
Sub site - mysite.com/part1/index.htm
Sub site - mysite.com/part2/index.htm

Each sub-folder will contain it's own index_htm_files folder with their own mouse-off/mouse-over images. These are not common across the total site but unique to each subfolder even though they're identical.

A way to avoid this is to create a dedicated navbar page to load into a placeholder on all the sub-folder pages:

Sub site - navbar/index.htm

The browser will cache the content. This also has the advantage of allowing a single amendment to the navbar sub-folder to go across all subfolders rather than needing to amend each one individually.