There will be two ways for photos to receive an alternative text. The first is from artificial intelligence. Instagram will use object recognition technology to automatically generation descriptions of photos that can be read by screen readers. Those AI produced descriptions will include lists of items that appear in a photo. While the text is likely to be a bit rudimentary in its descriptions, it's the only practical way to provide context for the billions of images hosted on the platform.
The second way for alternative text to be provided is directly from users. Instagram is introducing a custom alternative text box that will appear when you upload an image. You can use that space to provide a description of the image that will be more detailed and flowery than an automatically generated one.
Instagram said it has more plans to improve accessibility going forward, but didn't provide details regarding any future initiatives. Instagram's parent company Facebook has made a point of improving accessibility in recent years, adding alt text to its mobile apps and closed captions to live videos to help those with impairments use the platform.