Here's some information on using alternative text descriptions to make images posted online accessible:

Add descriptions to the alternative text fields when uploading images to CMS or Emma.

• Facebook: Uses automatic means to generate alternative text descriptions. This technology is still in its infancy, so to be more reliable it would be best to add descriptions manually. When posting an image you are prompted to “say something about his photo.” This is where to add the alternative text description. Learn more on Facebook's Help Center.
• Twitter: Allows image descriptions when posting from the web, iOS and Android devices. Learn more on Twitter.
• Instagram: On the page to share an Instagram post is an "advanced settings" tab, where an alternative text box for image descriptions can be found.