I’m a blind twitter user. There are a lot of us out there. Increase your ability to reach us and help us interact with your pictures, it’s really simple and makes a huge difference to our twitter experiance allowing us to see your images our way. Thanks for the description 😎 pic.twitter.com/hCsjoFdmev— Rob Long (@_Red_Long) January 3, 2018
Like you, I had no idea this was a thing! And why Twitter doesn't just set it up automatically, I don't know.
Basically go into your "Settings And Privacy."
Then click on "Accessibility."
Under that, turn on "Compose Image Descriptions."
Many celebrities have made comments saying that they've done it, including Chelsea Clinton and Lin-Manual Miranda. You can, too, and what a difference you'll make.