Thank you for the suggestion - a DLNA app on my phone did work, but I think it might have been smarter than the average client.
One interesting thing I have noticed is that if I go into Twonky via the web browser, I can look at the photos that way
I can also see that Twonly has assigned the photos new names, most of which begin “DLNA” then a load of random letters and finally “.jpg”
But all the photos I am having problems with seem to have the name “NON-DNLA” as their Twonly name, and all the photos are larger than 4000 pixels.
So this looks like a Twonky issue, but I wonder how I’m the only person who seems to have this, or are photos > 4000 pixels rare (especially as there is no TV with a resolution above 4000)
I COULD reduce the image size, of course, but am reluctant to do so in case I need to print them out.