Alt text should be added to all visual elements on the Libguide, including decorative elements, books from the catalog and infographics. If an image is merely decorative, you can add a null tag, which consists of two sets of quotation marks. If no title attribute is used, and the alt text is set to null (i.e. alt="") it indicates to assistive technology that the image can be safely ignored. This can be done in the case of headers and footers and any other part of a page that is only for design purposes.
In most cases you will be working with images and books from the catalog. In both these cases adding alt text is made easy by the software, which provides fields for the alt text when adding the content.
Alt text should be concrete and specific. If adding a photo to a guide, do not include in the alt text field the words, “photo of Mark Twain holding a kitten”. Just write “Mark Twain wearing a white suit holding a kitten”, so that the person experiencing the web page can have a mental image of the photo.
The Books from the Catalog dialog box has “Cover Art” as a default to add the the alt text field. In my opinion it is better to type in some concrete detail of the cover, if any. If it is just the book title on a plain background, then "Cover Art" is probably enough. If it has a photo of Mary Queen of Scots, I would add that.
Another category of image which needs alt text is any embedded chart, infographic or graph.