Consider that the pages are text files containing markup and links to named media files. When displayed DW interprets the markup and fetches the linked pages to build the final web page that is sent over the net to the reader. There is no hash or signature that identifies the media file, just the name.
If you want to keep old versions of media files you will have to adopt a naming scheme that incorporates a revision number or date in the file name. For instance my file bells:uss_pensylvania.jpg
will always be the latest version loaded. To version it I would need: