Thank you for your quick answers. Something did seem to break image previews in December as my theme (Customizr) stopped displaying previews of new uploads. But I upgraded to new versions of WP (3.8) and of my theme at the same time so I can't be sure where the problem lies. Based on what you say, it might be caused by WP's new library engine.
However, I can fix the image previews by enabling Retina screen functionality within my theme. Now I can see all my new image uploads -- but I am presumably also creating lots of different image sizes that I don't want since I am not developing a Retina-oprimised website.
I'm hoping that FRT will regenerate my missing thumbnails and also delete all the versions of my uploaded images which haven't been used anywhere. Anyway, I prefer to upload accurately sized images than to use the WP image editor.
But presumably some of the automatically scaled images which I don't physically add to my pages are called into action by my theme as it responds to smaller viewport sizes. How does FRT know that these "responsive" image variants will be needed on smaller screen devices?