You have the option to add alternative text or image descriptions to the photos you Tweet. Turn on the “compose image descriptions” setting in your Twitter account in order to reveal an option to add alt text to images in your tweets.
Twitter supports alternative text for static images
Currently, alternative text is not available for videos or animated GIFs, these will need a link to a description, or describe it in the Tweet message.
Consider providing an indication that a link in a tweet is a photo, video or audio file (e.g. [PIC], [VIDEO], [AUDIO]).
If your tweet links to photo, try to make your tweet act as a descriptive caption so it provides context for the content.
For images where you cannot add an image description, link back to a website page which has a tagged photo with a full image description. Or, send a follow-up tweet with a description.
Capitalize the first letter of each word: #AccessibleTweets.