(Not using the Pin Scheduler? Try this article for the Multi-Network Publisher
The alt text (or alt attribute) is an extra piece of information that is added to your photos that is not visible to most visitors. The primary purpose of the alt text is to describe the photo to visually impaired users so that they still have a good user experience when visiting your website, even if they are unable to see the photos.
The alt text will also show when your photo doesn’t load (due to page load errors) and provides context to search engines as to how your photo should be indexed.
Using alt text for Pinterest will also help boost your SEO ranking.
How to add Alt Text:
Visit the Pin Scheduler and open the Pin you want to edit (or create a new Pin draft).
Fill in the Pin data and type your alt text into the Alt text (optional) field, then click Schedule.
Pin Scheduler defaults and global controls (pin format, default extension account, board-duplicate warnings, and similar settings) now live under the schedule panel's three-dot (⋮) menu → Settings. This replaced the old "Global Pin settings" button that used to sit in the upper-right corner of the Pin Scheduler.