David Ok, I get what you are asking, although this should be a separate topic.
Cache currently uses
md5() encoding to encode the full image path into a file name. It has been considered storing full image paths in cache, like
_files/cache/images/path/to/image.320.jpg, but this requires creating nodes on server for each folder, and is potentially insecure. I might consider this for a future release.
You know you can just delete the entire image cache once in a while? It will just recreate cache for existing images.