Writing a description for an image is a breeze if you are limited by the number of Twitter characters or if you just want to explain the context, but adding it has been somewhat difficult so far. This is changing now that with Twitter you can now directly add a text-based description if you select an image without going into the image editing settings.
As of today, an alternative text box will be displayed on the Twitter mobile app and desktop Client, which you can use to quickly add an appropriate description for an image in your tweet. According to Twitter, this is an accessibility function with which "people who use screen readers can interpret images in tweets".
A black text box appears below the image under the heading "Add Description …" on mobile devices. You can tap on it and fill it with the description you have in mind. For the desktop client, just add and tap an image to find the "ALT" section above to add the description. The change applies to both regular tweets and direct messages that you send to a friend or follower.