I directed a colleague to look at photos at http://daviding.com/webphotos/201307n-Lougang/ , and when he selected a thumbnail, it led to a message of ...

Forbidden
You don't have permission to access this resource.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

I now noticed that the Slideshow works fine.

Suggestions?

I noticed a thread about symlink at https://forum.zenphoto.org/discussion/comment/1983607/#Comment_1983607 and tried that ... but it doesn't seem to help.

There are no symlinks involved here. That would be a standard image page url.

Is htaccess file present, is the rewrite base correctly set with /webphotos?

What do the logs say? Server PHP error log and the Zenphoto debug log?

Thanks for the guidance. Just to close the loop, I've migrated to a different hosting provider. Their tech support had a small issue during the migration, and was able to correct the issue.