How do I add functionality to images?

If you want to append HTML to images, you want to look inside the template files. Search for something like the_post_thumbnail function, and add your HTML right after the closing </a> tag (if there is one). You’d then need to style it with CSS so that it’s only visible upon hovering the image.

For example, in twentyfifteen theme (located inside /wp-content/twentyfifteen in the file system), content.php has a function (twentyfifteen_post_thumbnail) which is defined in inc/template-tags.php, and it’s used to display images on ‘single post’ pages as well as ‘post list’ pages.

If you’re after giving users an ability for album images to be shared (on Pinterest, etc.), it would be much easier to find a plugin that does this – there are quite a few, and you may just find one that has the effect you’re looking for.