Twitter improves accessibility

The compose image description option is available in Twitter's accessibility settings.

Twitter has begun to roll out a feature that allows images to be described when they are posted.

Once the feature has been enabled, users will see an option to add a description of the image.  They may use up to 420 characters. 

Visually impaired users will be able to access the description via assistive technologies such as braille displays.

Source and further information via the Twitter blog.