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.
You might have noticed in some websites that shows an icon on the side of the document or file link. PDF icon is most common and widget used icon on links directing to PDF. Now by using MimeTypes Link Icons plugin you can automatically convert your file links to their icons. The plugin covers a long list of supported file extensions where it would be showing the icons automatically.