Wanting to add a like and/or share button on the lightbox overlay with a default wordpress gallery. I know it can be done (more or less) with Nextgen but wanting to avoid having to redo all of the galleries on the site.
You will need to edit your lightbox template files. Are you using a plugin for the lightbox or is it included with the theme? Either way, you will need to locate the file and add your like button there.