Unfortunately, MobileSafari is making it harder and harder to actually save the image. If you gently press and hold, it'll give you the option to Save to Camera Roll. But with the advent of force touch, pressing too hard means that you'll end up navigating to a page with the image on it, losing your original page's state in the process.
It is possible to catch the force touch event, but as far as I can tell it overrides the regular touch event so all you can do is prevent the event and print "you pressed too hard, please release and try again, gently".
When was the last time you actually tried this? Because the reality doesn't match your description at all.
First off, you have to put a pretty decent amount of pressure on it to trigger the 3D touch behavior instead of the long press behavior.
Secondly, when you do trigger 3D touch, it goes into "peek" mode, and you have to keep pressing, and in fact press even harder, before it will "pop" and actually navigate to the image. In the "peek" mode simply swiping up will give you the Save Image action.
And if you're the Hulk and every single touch is a force touch, well, even gripping as hard as I can, it still forces the "peek" mode for a second or two before "popping".
Thank you; peek mode + swipe up does prompt to save the image and I did not know that. Based on user feedback, the "press-and-hold" method is the only one people commonly know about, and the major issue is that if you accidentally peek (or even imperceptibly drift into peek mode for a few ms) it won't work, and you won't get any feedback as to why.
It's handy to have a second way to trigger save, but having to include the explanation in order to teach users how to execute the maneuver is not great UX. "Press gently and hold to save. Or, if your phone supports Force Touch and has it enabled, press pretty hard (but not _too_ hard) and swipe up to save. If it doesn't work, try it again."
It is possible to catch the force touch event, but as far as I can tell it overrides the regular touch event so all you can do is prevent the event and print "you pressed too hard, please release and try again, gently".