Envira’s Downloads Addon will allow you to easily set up a download icon on your gallery and lightbox images as well as a Downloads All link for your gallery. Using the Password Protection Addon, you can also force a user to enter a password prior to allowing the download.
Gallery configurations:
4 column layout with 1px column gutter and margin below image. Lightbox theme Base (Dark) with Deeplinking, Supersize and Fullscreen enabled. Lightbox transition set on open to Zoom and on transition to Circular. Downloads All enabled below the gallery with force download enabled.