Alt text (alternative text) is a text string attribute for an image. Adding alt text to an image improves accessibility for readers who use screen readers. As of WordPress 4.7, alt text is no longer added automatically to images.
Before you start
You'll need a user account
1. Log in to your blog.
2. Navigate to Media > Library.
3. Click an image.
4. In Attachment Details, click Edit more details:
5. Scroll down to Alternative Text and update the field:
6. Click Update.