From the Twitter Blog:
Starting today, people using our iOS and Android apps can add descriptions — also known as alternative text (alt text) — to images in Tweets. With this update, we’re empowering everyone to ensure content shared on Twitter is accessible to the widest possible audience.
Enable this feature by using the compose image descriptions option in the Twitter app’s accessibility settings. The next time you add an image to a Tweet, each thumbnail in the composer will have an add description button. Tap it to add a description to the image. People who are visually impaired will have access to the description via their assistive technology (e.g., screen readers and braille displays). Descriptions can be up to 420 characters.
To ensure publishers and third-party clients also have the capability to add alt text to images, we’ve extended our platform products to both the REST API and Twitter Cards. We know this is especially important for specialized Twitter clients for the visually impaired such as EasyChirp, Chicken Nugget, and The Qube.