Adding ALT attribute (also called ALT tag) to a picture is to tag the image with text, which is convenient for search engines to grab the image descriptions and is helpful for SEO (search engine optimization). It also can meet the needs for reading aloud web pages for visually impaired users or busy people.
Step 1. In [Settings] > [Image Library] > Click the image name
Step 2. In the pop-up window, add or edit the ALT attribute for the image in the column, then click "OK" to complete the setting.