BlueA11y от ActiveB1t

Alternative (or alt) text is used to describe images for those who can't see the image, making them more accessible. This extension, based on Alt or Not for Twitter, shows which images are missing alternative text with a "No alt" label, and displays the alt text below the images when available.

You can choose to hide the repost button on posts with missing alt text, or even hide the complete post.

The extension includes additional options to check the text of posts for potential accessibility issues, giving suggestion on how to avoid those issues.