Many thanks for your quick and helpful reply.  Sorry if this was an inappropriate forum or I should have been more proactive in my investigations.

The problem was that the images were in /sites/default/files/gallery and Juicebox was trying to create the associated thumbnails in /sites/default/files/styles/thumbnail/public/gallery but the latter folder didn't exist.  Creating the folder fixed the problem.

Sorry, I should have said that I am using the Drupal Juicebox module - https://drupal.org/project/juicebox - with Juicebox Lite.

Hi
I have just installed the Juicebox Lite module in Drupal 7.  I generated a test gallery, but it isn't showing thumbnails: http://dev.hmssurprise.org/gallery
Can you help?

BTW, I plan to upgrade to the full version if I can get this working, as I wanted individual URLs for the images and more than 50 images.  Is the former possible in the Drupal 7 version of Juicebox?

Thanks,

Jane