How to enable the composition of image descriptions from twitter.com
- Click on the more icon and select Settings and privacy from the dropdown.
- Click Accessibility from the list of settings.
- Find the Compose image descriptions checkbox.
- Check the box to turn the setting on or off.
How to add image descriptions in Tweets from twitter.com
- Click on the Tweet compose button
- Attach your photo(s).
- To insert descriptive text, click Add description beneath the image
Jim Halpert from The Office (US Version) (c) 2007 NBC Universal Television. - Type your description of the image and click the Done button (The limit is 420 characters).
For tweets with multiple images, you must add each description separately.
Note: Image descriptions cannot be added to GIFs or videos. When using GIFs or videos, manually add a description in square brackets of the image at the end of your tweet.
Example:

Instructions for adding Alt-Text on an iPhone or Android mobile device are available in the Twitter Help Center.