Images must have alternative text (alt text). Alt text should be brief, descriptive and serve as a reasonable alternative to the image. This gives screen readers and those not able to load images the context for the image. Watch out for redundancy in your alt text — leave out phrases such as "Image of..." or similar extraneous content in your alt text.
In the absence of alt text, some screen readers will read out the filename of the image instead. Giving your image a meaningful name will prevent a screen reader from reading a confusing jumble of letters and numbers. It will also make it easier to find and reuse your images in LibGuides or the libraries' Drupal websites!
In a few rare instances, alt text can be redundant. In these cases, you can use empty alt text. This appears in HTML as alt="". Some text editors like Word may have a checkbox to mark the image as decorative instead. However, many website editors don't provide a way to mark an image as decorative. In most cases, it's better to provide alt text than not.
Examples of when it's ok not to provide alt text: