Several documents have been added to the Next-Gen CMS Documentation site related to best practices for using Media Library.
Documentation on file best practices covers what sort of information what may be necessary for creating a document on your site, making sure that file is accessible (if you absolutely need to upload it), and best practices for naming conventions.
Documentation on image best practices covers recommended image sizes based on content type and component, optimizing your images for the web, and best practices for naming conventions.
Tagging gives some best practices on tagging files and images in order to help you keep your site organized and make things easier to find in the future.
More details at: https://cms-docs.unl.edu/introduction-next-gen-cms/media-library/