Hermitage

You can set description for images. All you need is to set title attribute for links:

<a href="/path/to/thumbnail" title="Description here">
  <img src="/path/to/original/image" />
</a>

It is possible to do with Hermitage gem in different ways. The most simple way is just to pass title option to render_gallery_for method:

render_gallery_for @images, title: 'description'

where description is method of Image instance that returns its description.

Another way is to set this option in config/initializers/hermitage.rb config:

Hermitage.configure :images do
  title 'description'
end

Now you can customize CSS styles of this text:

hermitage.bottomPanel.text.styles = { fontSize: '20px', color: '#FAFAFA' }