I've just been looking to add image download links to NextGen Gallery. I had found the solution offered up by nbruley. (this topic: http://wordpress.org/support/topic/plugin-nextgen-gallery-adding-a-download-linkbutton?replies=15)
I've gotten so far as to download a zip file, but when opening it I am told that the file is not one that my computer understands...
Any help would be greatly appreciated!