WingerSpeed & volfro - thank you for your comments and pointers.
WingerSpeed - thank you also for the sterling work you have done to liberate JetPack from wp.com - why, though, reintroduce a dependency on wp.com through Photon? My use of images is modest enough so that any number crunching can easily be done locally without outsourcing the work.
I am not technical enough to add much to this discussion, except to say that I do make use of tiled galleries on a couple of my websites and, needing the same functionality on my local (development) MAMP server as well, I must continue to use the non-Photon earlier version of Slim Jetpack (2.3.3) which does the job adequately well and allows me to work without an internet connection.
It's only fair to point out that Slim Jetpack 2.7 (with Photon) displays tiled images perfectly well on my live websites, but I have down-versioned to Slim Jetpack 2.3.3 for continuity with my development environment.
Perhaps, WingerSpeed, you could add a user preference to disable Photon?