OK, I made Mail the default email reader. Then I quit Mail, and quit Photos. Then opened Mail - but it still did not show 'Mail' in the list.
So I closed the computer down, and restarted again.
Then it works! I can click on a photo and it sends as an email.
But, on the Photos page, it does not give an option to send at different sizes - until you get to the email, where the photo shows in the body of the email.
Then I noticed, 'Message size' and then on the right is a small box which says "Actual size'. Click on this and it gives the (hidden options) for emall, med and large. So that is good. (But it means I have to know how to use 'Mail', and learn all its little foibles!))
Thank you SO much for all your help, keg55. Marvellous!
Now what I want to know is, why are Apple FORCING me to use their 'Mail' app to email my photos?! It's a cheek! That is not democracy.
Why won't it work using Thunderbird?
How can we get Apple to allow different 'mail clients'?
If it had not been for your help, my photos would still be stuck in the new annoying 'Photos' - never to get out in any of my emails!