You can index all unreferenced images and clean them from your WordPress site in order to save space taken up all unreferenced images and make your website backup quick. Unreferenced images are the images which are not indexed in your WordPress site database and not used anywhere inside your content like blog posts, scripts. backup restore meta data etc.
Just like you pass tags in your WordPress blog posts; you can also tag images using a free WordPress plugin called WP Media Tagger. The plugin is extensively configurable and comes packed with a bunch of features enabling WordPress media tagging and search & media taxonomy etc.