Add “Email Image” button with Hyperlink for individual images [closed]

The easiest way is to create your own gallery template that has a form for each image (or perhaps a checkbox for each image, inside a form) and some handler code that takes the form submission and sends the emails. I’d do that as a plugin that hooks into the ‘ngg_render_template’ filter to load its own template.

You could start with NextGEN Download Gallery as a base, and change the download zip function to instead send your emails.