Thanks. Looks like I can't use Photon then. If I have to generate new slugs for each entry each time I need to change an image, then search engine referrals will result in 404s. I was lucky to change this slug (the entry's name was actually changed), but I'll still get 404s. I can't really do that for all future changes.
Adding a button to the wp admin to invalidate the cache for a site, would be very useful. Please consider some sort of invalidation routine either site wide or for specific images. Or possibly allow an option for a time expired cache (24 hrs, 48 hrs, etc.).