Add image to /_site/img/photos/original directory (do NOT create a subdirectory
here; photos in subdirectories don't seem to get propagated to the
large and small directories).
Run bundle exec jekyll serve
/_site/img/photos/large and /_site/img/photos/small should now have
corresponding images to the ones that you placed in the original
directory.
Copy the newly updated /_site/img/photos directory, and paste it into
/img/ directory (replacing the old /img/photos directory).