OK, so on my development system I restored my site from a backup taken when it was running Kunena 1.5.5 and tried using an iPhone and an iPad to insert an image into a post. The result:
it wouldn't work.
So I went back to my users who'd reported the problem to check and they honestly couldn't swear they'd tried to do this until I'd upgraded to 1.7.2. SO it looks like it's not a Kunena problem to me.
However, I do want to provide my users a good experience regardless of the platform they use to access my site, so after some further experimentation I have come up with a solution/workaround that is really simple to implement, does not require any effort from either myself or the Kunena team, and leaves the choice fully in the hands of my users.
The solution is
not to use the Safari browser on iOS to access the Kunena forum, but to use a different browser called
iCabMobile which allows uploading of images to the forum. And to prove it, here are a couple of screenshots from an iPad doing just that with both 1.5.5 and 1.7.2. I don't know if other Safari alternatives will work - but it definitely does with iCabMobile. (I also have no affiliation to the authors of iCabMobile by the way.)
I think this goes some way to dispel the myth that iOS is special somehow: it's simply a case of Apple not building the facility into Safari.
I now consider this a closed issue - thanks for your help and support.