What is the "Alt text" of my images used for?
Alt Text provides users and search engines with information about your images. Search engine robots are unable to “read” images - only text.
To add Alt text to an image: Click the image to select it > from the settings window, under Alt Text, enter the Alt text.
For Gallery images, the text in the Title field will function as the Alt text. Google will also take the image description into account.