Alternative text (or alt text) is a method for describing an image when it cannot be processed visually. For instance, a blind or visually impaired user utilizing a screen reader with a speech synthesizer or braille display will be able to absorb all the content on your page.
Select your image and click the Settings button (1) to open the image control panel on the left side of your screen.
Navigate to the image control panel and enter your description into the Alt Text field (2) at the bottom of the panel.
Tips for Writing Effective Alt Text
Make it meaningful
Your alt text is a replacement for your image, do your best to describe the picture and convey it's meaning.
Keep it simple
Most alt text should be easily described in under a sentence.
Don't include "image of" or "picture of"
Since the purpose of alt text is to convey the content of the image, it should be easily discernible to most users that the text is a descriptor of an image.
Not Good: Library
Good: The Sainte-Geneviève Library
Better: Interior view of the Sainte-Geneviève Library