Start by tapping the Tweet compose icon and attach your photo(s).
Note: For detailed instructions about adding photos to your Tweets, read this article.
On the image, tap the +Alt button to insert descriptive text.
Type your description of the image and tap Done. Tap the description again to edit it prior to posting the Tweet. (The limit is 1000 characters.)
You can add a description to each image in a Tweet.
Note: Image descriptions cannot be added to videos.