So, I don't have my handi-dandi reference tool with me at the moment, but you could take a look at a plugin called Zen Cart overrides. (I think that's what it is called.) It is a small downloadable html/java or activex document that shows what can be overridden and where the file(s) need to be stored. If, however, you find that a specific file can not be overriden, but there is an attribute (perhaps template related) that can and should be overriden, then you may need to custom code it to include the template override call for the desired "attribute" (yes that would be a core file/related mod) obviously would suggest providing comments in the file(s) so when updating will make sense.
Or....
Could use a responsive template and maybe an image handler to reduce file sizes. Your choice of course.