Alternative Text is a descriptive attribute that you can associate with images. It is meant to serve the same purpose by conveying the same information as the image in situations where the image is not available to the reader, or if the reader is visually impaired and is using a screen reader. Learn more about web accessibility requirements at the Harvard Guidelines site. Read more about Adding Alt Text
Private files are restricted to the Harvard Community or optionally just members of your site. Site visitors from the Harvard community will be asked to login with a valid HUID to access your private files also any member of your site will have access when logged into the site. Private files is the only way to upload a file that is not available to the public, they will stay private independent of site visibility.